INSECTS***


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[Illustration: NATURE SERIES]




ORIGIN AND METAMORPHOSES OF INSECTS.

[Illustration]

Nature Series

ON THE ORIGIN AND METAMORPHOSES OF INSECTS

BY

Sir John Lubbock, Bart., M.P., F.R.S., D.C.L., LL.D.

Principal of the London Working Men's College; President of the London
Chamber of Commerce; and Vice-Chairman of the London County Council

With Numerous Illustrations







London Macmillan and Co. and New York 1890

The Right of Translation and Reproduction is Reserved

Richard Clay and Sons, Limited, London and Bungay.

First Edition 1873. Reprinted 1874. New Edition 1890.




PREFACE.


For some years, much of my leisure time has been devoted to the study of
the anatomy, development, and habits of the Annulosa, and especially of
Insects, on which subjects I have published various memoirs, chiefly in
the Transactions of the Royal, Linnaean, and Entomological Societies: of
these papers I subjoin a list. Although the details, of which these
memoirs necessarily for the most part consist, offer little interest,
excepting to those persons who are specially devoted to Entomology,
still there are portions which, having reference to the nature of
metamorphoses and to the origin of insects, are of a more general
character. I have also briefly referred to these questions in a
Monograph of the Collembola and Thysanura, recently published by the Ray
Society, and in the Opening Address to the Biological Section of the
British Association at Brighton in 1872. Under these circumstances, it
has been suggested to me that a small volume, containing, at somewhat
greater length, in a more accessible form, and with the advantage of
illustrations, the conclusions to which I have been led on this
interesting subject, might not be altogether without interest to the
general reader. The result, which has already appeared in the pages of
_Nature_, is now submitted to the public, with some additions. I am well
aware that it has no pretence to be in any sense a complete treatise;
that the subject itself is one as to which our knowledge is still very
incomplete, and on which the highest authorities are much divided in
opinion. Whatever differences of opinion, however, there may be as to
the views here put forward, the facts on which they are based will, I
believe, be found correct. On this point I speak with the more
confidence, on account of the valuable assistance I have received from
many friends: to Mr. and Mrs. Busk and Dr. Hooker I am especially
indebted.

The papers above referred to are as follows:--

   1. _On Labidocera._--Annals and Magazine of Natural History,
        vol. xi., 1853.

   2. On Two New Sub-genera of Calanidae.--Annals and Magazine of
        Natural History, vol. xii., 1853.

   3. On Two New Species of Calanidae.--Annals and Magazine of
        Natural History, vol. xii., No. lxvii., 1853.

   4. On Two New Species of Calanidae.--Annals and Magazine of
        Natural History, vol. xii., No. lxix., 1853.

   5. On some Arctic Calanidae.--Annals and Magazine of Natural
        History, 1854.

   6. On the Freshwater Entomostraca of South
        America.--Transactions of the Entomological Society, vol.
        iii., 1855.

   7. On some New Entomostraca.--Transactions of the
        Entomological Society, vol. iv., 1856.

   8. On some Marine Entomostraca found at Weymouth.--Annals and
        Magazine of Natural History, vol. xx., 1857.

   9. On the Respiration of Insects.--Entomological Annual, 1857.

  10. An Account of the Two Methods of Reproduction in
         _Daphnia_.--Transactions of the Royal Society, 1857.

  11. On the Ova and Pseudova of Insects.--Transactions of the
        Royal Society, 1858.

  12. On the Arrangement of the Cutaneous Muscles of _Pygaera
        Bucephala_.--Linnean Society's Transactions, vol. xxii., 1858.

  13. On the Freshwater Entomostraca of South
        America.--Entomological Society's Transactions, 1858.

  14. On _Coccus Hesperidum_.--Royal Society Proceedings, vol.
        ix., 1858.

  15. On the Distribution of Tracheae in Insects.--Linnean
        Society's Transactions, vol. xxiii., 1860.

  16. On the Generative Organs and on the Formation of the Egg
        in Annulosa. Transactions of the Royal Society, 1861.

  17. On _Sphaerularia Bombi._--Natural History Review, 1861.

  18. On some Oceanic Entomostraca.--Linnean Society's
        Transactions, vol. xxiii., 1860.

  19. On the Thysanura. Part 1.--Linnean Society's Transactions,
        1862.

  20. On the Development of Lonchoptera.--Entomological
        Society's Transactions, 1862.

  21. On the Thysanura. Part 2.--Linnean Society's Transactions,
        1862.

  22. On the Development of Chloeon. Part 1.--Linnean Society's
        Transactions, 1863.

  23. On Two Aquatic Hymenoptera.--Linnean Society's
        Transactions, 1863.

  24. On some little-known Species of Freshwater
        Entomostraca.--Linnean Society's Transactions, vol. xxiv.,
        1863.

  25. On _Sphaerularia Bombi_.--Natural History Review, 1864.

  26. On the Development of Chloeon. Part 2.--Linnean Society's
        Transactions, 1865.

  27. Metamorphoses of Insects.--Journal of the Royal
        Institution, 1866.

  28. On _Pauropus_.--Linnean Society's Transactions, 1866.

  29. On the Thysanura. Part 3.--Linnean Society's Transactions,
        1867.

  30. Address to the Entomological Society.--Entomological
        Society's Transactions, 1867.

  31. On the Larva of Micropeplus Staphilinoides.--Entomological
        Society's Transactions, 1868.

  32. On the Thysanura. Part 4.--Linnean Society's Transactions,
        1869.

  33. Addresses to the Entomological Society.--Entomological
        Society's Transactions, 1867-1868.

  34. On the Origin of Insects.--Journal of the Linnean Society,
        vol. xi.

  35. Opening Address to the Biological Section of the British
        Association.--British Association Report, 1872.

  36. Observations on Ants, Bees, and Wasps. Part 1.--Journal of
        the Linnean Society, 1873.

  37. On British Wild Flowers considered in relation to Insects,
        1874.

  38. Observations on Ants, Bees, and Wasps. Part 2.--Journal of
        the Linnean Society, 1874.

  39. Observations on Ants, Bees, and Wasps. Part 3.--Journal of
        the Linnean Society, 1875.

  40. Observations on Ants, Bees, and Wasps. Part 4.--Journal of
        the Linnean Society, 1877.

  41. On some Points in the Anatomy of Ants.--Quekett Lecture,
        1877.--Microscopical Journal.

  42. On the Colors of Caterpillars.--Entomological Society's
        Transactions, 1878.

  43. Observations on Ants, Bees, and Wasps. Part 5.--Journal of
        the Linnean Society, 1878.

  44. Observations on Ants, Bees, and Wasps. Part 6.--Journal of
        the Linnean Society, 1879.

  45. On the Anatomy of Ants.--Linnean Society's Transactions,
        1880.

  46. Observations on Ants, Bees, and Wasps. Part 7.--Journal of
        the Linnean Society, 1880.

  47. Observations on Ants, Bees, and Wasps. Part 8.--Journal of
        the Linnean Society, 1881.

  48. On Fruits and Seeds.--Journal of the Royal Institution,
        1881.

  49. Observations on Ants, Bees, and Wasps. Part 9.--Journal of
        the Linnean Society, 1881.

  50. On the Limits of Vision among some of the lower
        Animals.--Journal of the Linnean Society, 1881.

  51. Observations on Ants, Bees, and Wasps. Part 10.--Journal
        of the Linnean Society, 1882.




CONTENTS.


  CHAPTER I.

  THE CLASSIFICATION OF INSECTS.

  Introduction.--Stages in the Life of an Insect.--Classification
    of Insects.--Characters derived from the Wings; from the parts
    of the Mouth; from the Metamorphoses.--The Classes of Insects:
    Hymenoptera, Strepsiptera, Coleoptera, Euplexoptera,
    Orthoptera, Thysanoptera, Neuroptera, Trichoptera, Diptera,
    Aphaniptera, Heteroptera, Homoptera, Lepidoptera          _page_ 1-26


  CHAPTER II.

  THE INFLUENCE OF EXTERNAL CONDITIONS ON THE FORM AND STRUCTURE
  OF LARVAE.

  Larvae depend partly on the group to which they
    belong.--Wood-eating Larvae.--Larvae of Lamellicorns.--Larvae
    depend also in part on mode of life.--Larvae of Hymenoptera, of
    _Sirex_; of _Tenthredo_; of Ichneumons; of Bees.--Rudimentary
    legs of Bee Embryo.--Beetles, _Weevils_, _Scolytus_,
    _Crioceris_, _Sitaris_, Metamorphoses of Pteromalidae.
    _Platygaster_, _Polynema_.--Influence of external
    conditions.--Developmental and adaptive Metamorphoses    _page_ 27-41


  CHAPTER III.

  THE NATURE OF METAMORPHOSES.

  The life history of an Insect must be considered as a
    whole.--Vagueness of the term Larva.--Some larvae much more
    advanced than others.--Organs develope in different order, in
    different groups.--Suppressed stages.--Apod condition of
    _Phryganea_; of _Aphis_; of
    _Chrysopa_.--Libellulidae.--_Donacia_.--Spiders.--Myriapods.--Apod
    stage of Homomorphous Insects once probably longer than
    now.--Suppression of embryonic stages.--Metamorphoses of
    Hydroida, Crustacea, Isopods, and
    Amphipods.--Echinoderms.--Variations in development induced by
    the influence of external conditions.                    _page_ 41-62


  CHAPTER IV.

  THE ORIGIN OF METAMORPHOSES.

  Origin of Metamorphoses.--Views of Messrs. Kirby and
    Spence.--Nature of the question.--Young animals often more
    similar than mature.--Views of Darwin, Herbert Spencer,
    Johannes Mueller, Fritz Mueller, and Agassiz.--Effect of size of
    egg.--Insects leave the egg in a more or less developed
    condition.--Consideration of pupal condition.--Quiescence of
    pupa.--Period of quiescence at each moult.--Changes not so
    abrupt as generally supposed.--Change in
    mouth-parts.--Difficulty in reference to Darwinian
    theory.--Mouth-parts of _Campodea_ and Collembola, as
    intermediate between the mandibulate and haustellate
    types.--Change in mouth-parts as connected with pupal
    conditions.--Origin of wings.--Use of wings under
    water.--Connection of metamorphoses with alternation of
    generations.--Parthenogenetic larvae of _Cecidomyia_.--In
    alternation of generations one form always agamic.--Dimorphism
    and Dieidism.--Summary and Conclusions                   _page_ 62-81


  CHAPTER V.

  THE ORIGIN OF INSECTS.

  The Origin of Insects.--Mistaken views of Darwinian
    theory.--Natural selection a _vera causa_.--Application of
    Darwin's views to Insects.--Similarity of young Crustacea as
    compared with mature forms; ditto in Insects.--Type of
    Insecta.--Two principal types of larvae: Hexapod and
    Apod.--Conclusions to be drawn from them.--_Campodea_ the modern
    representative of the Insect-stock.--_Campodea_, perhaps
    derived from Tardigrade.--Vermiform or Apod type of
    larva.--Views of Fritz Mueller, Brauer, and Packard.--Represents
    a still earlier ancestor.--Modern representatives.--_Notommata_,
    _Albertia_, _Lindia_.--Earlier forms difficult to trace.--Lowest
    forms of animal life.--Yolk-segmentation.--Embryology and
    Evolution.--Light thrown by the evolution of the individual on
    that of the species                                     _page_ 82-108

  DESCRIPTION OF THE PLATES.


  PLATE I. p. 7.

  FIG.

  1. Cricket. Westwood, Intro. to the Modern Classification of
       Insects, vol. i. p. 440.

  2. Earwig. Westwood, loc. cit. vol. i. p. 399.

  3. _Aphis_. Packard, Guide to the Study of Insects, pp. 521, 522.

  4. _Scolytus_. Westwood, loc. cit. vol. i. p. 350.

  5. _Anthrax_. Westwood, loc. cit. vol. ii. p. 538.

  6. _Balaninus_.

  7. _Cynips_. Westwood, loc. cit. vol. ii. p. 121.

  8. Ant (_Formica_). Westwood, loc. cit. vol. ii. p. 218.

  9. Wasp. Ormerod, Nat. Hist. of Wasps, pl. i. fig. 1.


  PLATE II. p. 8.

  FIG.

  1. Larva of Cricket. Westwood, loc. cit. vol. i. p. 440.

  2. Larva of _Aphis_. Packard, loc. cit. pp. 521, 522.

  3. Larva of Earwig. Westwood, loc. cit. vol. i. p. 399.

  4. Larva of _Scolytus_. Westwood, loc. cit. vol. i. p. 350.

  5. Larva of _Anthrax_. Westwood, loc. cit. vol. ii. p. 546.

  6. Larva of _Balaninus_.

  7. Larva of _Cynips_. Westwood, loc. cit. vol. ii. p. 121.

  8. Larva of Ant (_Formica_). Westwood, loc. cit. vol. ii. p. 226.

  9. Larva of Wasp. Newport, Art. Insecta, Todd's Cycl. Anat. and
       Phys., p. 871.


  PLATE III. p. 14.

  FIG.

  1. _Chloeon_. Linn. Trans. 1866.

  2. _Meloe_. Spry and Shuckard, Coleoptera Delineated, pl. 56.

  3. _Calepteryx_.

  4. _Sitaris_. Spry and Shuckard, loc. cit. pl. 56.

  5. _Campodea_. Suites a Buffon. Apteres.

  6. _Acilius_. Westwood, loc. cit. vol. i. p. 100.

  7. _Termes_. Westwood, loc. cit. vol. ii. p. 12.

  8. _Stylops_. Duncan, Met. of Insects, p. 387; Packard, p. 482.

  9. _Thrips_. Westwood, loc. cit. vol. ii. p. 1.


  PLATE IV. p. 15.

  FIG.

  1. Larva of _Chloeon_. Linn. Trans. 1863.

  2. Larva of _Meloe_. Chapuis and Candeze, Mem. Soc. Roy. Liege,
       1853, pp. 1, 7.

  3. Larva of _Calepteryx_. Dufour, Ann. Sci. Nat. 1852.

  4. Larva of _Sitaris_. Duncan, Met. of Insects, p. 309.

  5. Larva of _Campodea_. Gervais' Suites a Buffon. Apteres.

  6. Larva of _Acilius_. Westwood, loc. cit. vol. i. p. 100.

  7. Larva of _Termes_. Duncan, loc. cit. p. 348.

  8. Larva of _Stylops_. Westwood, Trans. Ent. Soc. 1839, vol. ii.
       pl. xv. fig. 13a.

  9. Larva of _Thrips_. Westwood, loc. cit. vol. ii. p. i.


  PLATE V. p. 99.

  FIG.

  1-5. _Protamoeba_.

  6-9. _Protamyxa aurantiaca_. Haeckel Beit. zur. Monog. der
         Moneren, pl. 1.

  10-18. _Magosphoera planula_. Haeckel, loc. cit. pl. v.


  PLATE VI. p. 105.

  FIG.

  1-4. Yolk-segmentation in _Laomedea_. After Allman. Mon. of
         Tubularian Hydroids. Ray Society.

  5-9. Yolk-segmentation in _Filaria_. After Van Beneden. Mem. sur
         les Vers Intestinaux.

  10-13. Yolk-segmentation in _Echinus_. After Derbes. Ann. des.
           Sci. Nat. 1847.

  14-17. Yolk-segmentation in _Lacinularia_. After Huxley. J. of
           Mic. Sci. 1853.

  18-21. Yolk-segmentation in _Purpura_. After Koren and
           Danielssen. Ann. des. Sci. Nat. 1853.

  22-24. Yolk-segmentation in _Amphioxus_. After Haeckel.
           Naturliche Schoepfungsgeschichte, pl. x.

  25-29. Yolk-segmentation in Vertebrate. After Allen Thompson.
           Art. Ovum. Cyclop. of Anatomy and Physiology.


  DESCRIPTION OF THE FIGURES.


  FIG. 1. Larva of the Cockchafer (_Melolontha_)

   2. Larva of _Cetonia_.

   3. Larva of _Trox_.

   4. Larva of _Oryctes_.

   5. Larva of _Aphodius_.

   6. Larva of _Lucanus_.

   7. Larva of _Brachytarsus_.

   8. Larva of _Crioceris_.

   9. Larva of _Sitaris humeralis_.

  10. Larva of _Sitaris humeralis_, in the second stage.

  11. Larva of _Sitaris humeralis_, in the third stage.

  12. Larva of _Sitaris humeralis_, in the fourth stage.

  13. Pupa of _Sitaris_.

  14. Larva of _Sirex_.

  15. Egg of _Rhynchites_, showing the parasitic larva.

  16. The parasitic larva, more magnified.

  17. Egg of _Platygaster_.

  18. Egg of _Platygaster_, showing the central cell.

  19. Egg of _Platygaster_, after the division of the central cell.

  20. Egg of _Platygaster_, more advanced.

  21. Egg of _Platygaster_, more advanced.

  22. Egg of _Platygaster_, showing the rudiment of the embryo.

  23. Larva of _Platygaster_.--_mo_, mouth; _a_, antenna; _kf_,
        hooked feet; _r_, toothed process; _lfg_, lateral
        process; _f_, branches of the tail.

  24. Larva of another species of _Platygaster_. (The letters
        indicate the same parts as in the preceding figure.)

  25. Larva of a third species of _Platygaster_. (The letters
        indicate the same parts as in the preceding figure.)

  26. Larva of _Platygaster_ in the second stage.--_mo_, mouth;
        _slkf_, oesophagus; _gsae_, supra-oesophagal ganglion;
        _lm_, muscles; _bsm_, nervous system; _gagh_, rudiments
        of the reproductive glands.

  27. Larva of _Platygaster_ in the third stage.--_mo_, mouth;
        _ma_, mandibles; _gsae_, supra-oesophagal ganglion; _slk_,
        oesophagus; _ag_, ducts of the salivary glands; _bnm_,
        ventral nervous system; _sp_, salivary glands; _msl_,
        stomach; _im_, imaginal discs; _tr_, tracheae; _fk_, fatty
        tissue; _ed_, intestine; _ga_, rudiments of reproductive
        organs; _ew_, wider portion of intestine; _ao_, posterior
        opening.

  28. Embryo of _Polynema_.

  29. Larva of _Polynema_.--_asch_, rudiments of the antennae;
        _flsch_, of the wings; _bsch_, of the legs; _vfg_,
        lateral projections; _gsch_, rudiments of the ovipositor;
        _fk_, fatty tissue.

  30. Egg of _Phryganea_ (Mystacides).--_A_^1, mandibular segment;
        _C_^1-_C_^5, maxillary, labial, and three thoracic
        segments; _D_, abdomen.

  31. Egg of _Phryganea_ somewhat more advanced.--_b_, mandibles;
        _c_, maxillae; _cfs_, rudiments of the three pairs of
        legs.

  32. Egg of _Pholcus opilionides_, showing the Protozonites.

  33. Embryo of _Julus_.

  34. Colony of _Bougainvillea fruticosa_, natural size, attached
        to the underside of a piece of floating timber.

  35. Portion of the same, more magnified.

  36. The Medusa from the same species.

  37. Larva of Prawn, Nauplius stage.

  38. Larva of Prawn, more advanced, Zoea stage.

  39. Larva of Echino-cidaris oequituberculata seen from above x6/10.

  40. Larva of _Echinus_ x 100.--_A_, front arm; _F_, arms of the
        mouth-process; _B_, posterior side arm; _E_^1, accessory
        arm of the mouth-process; _a_, mouth; _a_^1, oesophagus;
        _b_, stomach; _b_^1, intestine; _o_, posterior orifice;
        _d_, ciliated bands; _f_, ciliated epaulets; _c_, disc of
        future _Echinus_.

  41. _Comatula rosacea_.

  42. Larva of _Comatula rosacea_.

  43. Larva of _Comatula rosacea_, more advanced.

  44. Larva of _Comatula rosacea_, in the Pentacrinus state.

  45. Larva of Starfish (Bipinnaria), x 100.

  46. Larva of Starfish (Bipinnaria), x 100, seen from the
  side.--_a_, mouth; _b_, oesophagus; _c_, stomach; _c_^1,
  intestine.

  47. Larva of another Bipinnaria, showing the commencement of
  the Starfish.--_g_, canal of the ciliated sac; _i_, rudiments
  of tentacles; _d_, ciliated band.

  48. Larva of Moth (_Agrotis_).

  49. Larva of Beetle (_Haltica_).

  50. Larva of Saw-fly (_Cimbex_).

  51. Larva of _Julus_.

  52. _Agrotis suffusa_.

  53. _Haltica_.

  54. _Cimbex_.

  55. _Julus_.

  56. Tardigrade.

  57. Larva of _Cecidomyia_.

  58. _Lindia torulosa_.

  59. _Prorhynchus stagnalis_.

  60. Egg of Tardigrade.

  61. Egg of Tardigrade, after the yolk has subdivided.

  62. Egg of Tardigrade, in the next stage.

  63. Egg of Tardigrade, more advanced.





ON THE ORIGIN AND METAMORPHOSES OF INSECTS.




CHAPTER I.

  _THE CLASSIFICATION OF INSECTS._


About forty years ago the civil and ecclesiastical authorities of St.
Fernando in Chili arrested a certain M. Renous on a charge of
witchcraft, because he kept some caterpillars which turned into
butterflies.[1] This was no doubt an extreme case of ignorance; it is
now almost universally known that the great majority of insects quit the
egg in a state very different from that which they ultimately assume;
and the general statement in works on entomology has been that the life
of an insect may be divided into four periods.

Thus, according to Kirby and Spence,[2] "The states through which
insects pass are four: the _egg_, the _larva_, the _pupa_, and the
_imago_." Burmeister,[3] also, says that, excluding certain very rare
anomalies, "we may observe four distinct periods of existence in every
insect,--namely, those of the egg, the larva, the pupa, and the imago,
or perfect insect." In fact, however, the various groups of insects
differ widely from one another in the metamorphoses they pass through:
in some, as in the grasshoppers and crickets, the changes consist
principally in a gradual increase of size, and in the acquisition of
wings; while others, as for instance the common fly, acquire their full
bulk in a form very different from that which they ultimately assume,
and pass through a period of inaction in which not only is the whole
form of the body altered, not only are legs and wings acquired, but even
the internal organs themselves are almost entirely disintegrated and
re-formed. It will be my object, after having briefly described these
changes, to throw some light on the causes to which they are due, and on
the indications they afford of the stages through which insects have
been evolved.

The following list gives the orders or principal groups into which the
Class Insecta may be divided. I will not, indeed, here enter upon my own
views, but will adopt the system given by Mr. Westwood in his excellent
"Introduction to the Modern Classification of Insects," from which also,
as a standard authority, most of the figures on Plates I. to IV., when
not otherwise acknowledged, have been taken. He divides insects into
thirteen groups, and with reference to eight of them it may be said that
there is little difference of opinion among entomologists. These orders
are by far the most numerous, and I have placed them in capital
letters. As regards the other five there is still much difference of
opinion. It must also be observed that Prof. Westwood omits the
parasitic Anoplura, as well as the Thysanura and Collembola.


ORDERS OF INSECTS ACCORDING TO WESTWOOD.

   1. HYMENOPTERA   Bees, Wasps, Ants, &c.
   2. STREPSIPTERA  _Stylops_, _Zenos_, &c.
   3. COLEOPTERA    Beetles.
   4. EUPLEXOPTERA  Earwigs.
   5. ORTHOPTERA    Grasshoppers, Crickets, Cockroaches, &c.
   6. THYSANOPTERA  _Thrips_.
   7. NEUROPTERA    _Ephemeras_, &c.
   8. TRICHOPTERA   _Phryganea_.
   9. DIPTERA       Flies and Gnats.
  10. APHANIPTERA   Fleas.
  11. HETEROPTERA   Bugs.
  12. HOMOPTERA     _Aphis_, _Coccus_, &c.
  13. LEPIDOPTERA   Butterflies and Moths.

Of these thirteen orders, the eight which I have placed in capital
letters--namely the first, third, fifth, seventh, ninth, eleventh,
twelfth, and thirteenth, are much the most important in the number and
variety of their species; the other five form comparatively small
groups. The Strepsiptera are minute insects, parasitic on Hymenoptera:
Rossi, by whom they were discovered, regarded them as Hymenopterous;
Lamarck placed them among the Diptera; by others they have been
considered to be most closely allied to the Coleoptera, but they are now
generally treated as an independent order.

The Euplexoptera or Earwigs are only too familiar to most of us. Linnaeus
classed them among the Coleoptera, from which, however, they differ in
their transformations. Fabricius, Olivier, and Latreille regarded them
as Orthoptera; but Dr. Leach, on account of the structure of their
wings, considered them as forming the type of a distinct order, in which
view he has been followed by Westwood, Kirby, and many other
entomologists.

The Thysanoptera, consisting of the Linnaean genus _Thrips_, are minute
insects well known to gardeners, differing from the Coleoptera in the
nature of their metamorphoses, in which they resemble the Orthoptera and
Hemiptera. The structure of the wings and mouth-parts, however, are
considered to exclude them from these two orders.

The Trichoptera, or Caddis worms, offer many points of resemblance to
the Neuroptera, while in others they approach more nearly to the
Lepidoptera. According to Westwood, the genus _Phryganea_ "forms the
connecting link between the Neuroptera and Lepidoptera."

The last of these small aberrant orders is that of the Aphaniptera,
constituted for the family Pulicidae. In their transformations, as in
many other respects, they closely resemble the Diptera. Strauss
Durckheim indeed said that "_la puce est un diptere sans ailes_."
Westwood, however, regards it as constituting a separate order.

As indicated by the names of these orders, the structure of the wings
affords extremely natural and convenient characters by which the various
groups may be distinguished from one another. The mouth-parts also are
very important; and, regarded from this point of view, the Insecta have
been divided into two series--the Mandibulata and Haustellata, or
mandibulate and suctorial groups, between which, as I have elsewhere
shown,[4] the Collembola (_Podura_, _Smynthurus_, &c.) occupy an
intermediate position. These two series are:--

MANDIBULATA.

  Hymenoptera.
  Strepsiptera.
  Coleoptera.
  Euplexoptera.
  Orthoptera.
  Trichoptera?
  Thysanoptera?

HAUSTELLATA.

  Lepidoptera.
  Diptera.
  Aphaniptera.
  Hemiptera.
  Homoptera.

Again--and this is the most important from my present point of
view--insects have sometimes been divided into two other series,
according to the nature of their metamorphoses: "Heteromorpha," to use
the terminology of Prof. Westwood,[5] "or those in which there is no
resemblance between the parent and the offspring; and Homomorpha, or
those in which the larva resembles the imago, except in the absence of
wings. In the former the larva is generally worm-like, of a soft and
fleshy consistence, and furnished with a mouth, and often with six short
legs attached in pairs to the three segments succeeding the head. In the
Homomorpha, including the Orthoptera, Hemiptera, Homoptera, and certain
Neuroptera, the body, legs, and antennae are nearly similar in their form
to those of the perfect insect, but the wings are wanting."

HETEROMORPHA.

  Hymenoptera.
  Strepsiptera.
  Coleoptera.
  Trichoptera.
  Diptera.
  Aphaniptera.
  Lepidoptera.

HOMOMORPHA.

  Euplexoptera.
  Orthoptera.
  Hemiptera.
  Homoptera.
  Thysanoptera.
  Neuroptera.

But though the Homomorphic insects do not pass through such striking
changes of form as the Heteromorphic, and are active throughout life,
still it was until within the last few years generally (though
erroneously) considered, that in them, as in the Heteromorpha, the life
fell into four distinct periods; those of (1) the egg, (2) the larva,
characterized by the absence of wings, (3) the pupa with imperfect
wings, and (4) the imago, or perfect insect.

I have, however, elsewhere[6] shown that there are not, as a matter of
fact, four well-marked stages, and four only, but that in many cases the
process is much more gradual.

The species belonging to the order Hymenoptera are among the most
interesting of insects. To this order belong the gallflies, the
sawflies, the ichneumons, and, above all, the ants and bees. We are
accustomed to class the Anthropoid apes next to man in the scale of
creation, but if we were to judge animals by their works, the chimpanzee
and the gorilla must certainly give place to the bee and the ant. The
larvae of the sawflies, which live on leaves, and of the Siricidae or
long-tailed wasps, which feed on wood, are very much like caterpillars,
having three pairs of legs, and in the former case abdominal pro-legs
as well: but in the great majority of Hymenoptera the larvae are legless,
fleshy grubs (Plate II., Figs. 7-9); and the various modes by which the
females provide for, or secure to, them a sufficient supply of
appropriate nourishment constitutes one of the most interesting pages of
Natural History.

The species of Hymenoptera are very numerous; in this country alone
there are about 3,000 kinds, most of which are very small. In the pupa
state they are inactive, and show distinctly all the limbs of the
perfect insect, encased in distinct sheaths, and folded on the breast.
In the perfect state they are highly organized and very active. The
working ants and some few species are wingless, but the great majority
have four strong membranous wings, a character distinguishing them at
once from the true flies, which have only one pair of wings.

The saw-flies are so called because they possess at the end of the body a
curious organ, corresponding to the sting of a wasp, but which is in the
form of a fine-toothed saw. With this instrument the female sawfly cuts
a slit in the stem or leaf of a plant, into which she introduces her
egg. The larva much resembles a caterpillar, both in form and habits. To
this group belongs the <DW65>, or black caterpillar of the turnip, which
is often in sufficient numbers to do much mischief. Some species make
galls, but the greater number of galls are formed by insects of another
family, the Cynipidae.

[Illustration: PLATE I.[7]--MATURE INSECTS.

Fig. 1, Cricket; 2, Earwig; 3, _Aphis_; 4, _Scolytus_; 5, Anthrax;
6, _Balaninus_; 7, _Cynips_; 8, Ant; 9, Wasp.]

[Illustration: PLATE II.--LARVAE OF THE INSECTS REPRESENTED ON PLATE I.

Fig. 1, Larva of Cricket; 2, Larva of Aphis; 3, Larva of Earwig; 4,
Larva of _Scolytus_ (Beetle); 5, Larva of _Anthrax_ (Fly); 6, Larva of
_Balaninus_ (Nut Weevil); 7, Larva of _Cynips_; 8, Larva of Ant;
9, Larva of Wasp.]

In the Cynipidae (Plate I., Fig. 7) the female is provided with an organ
corresponding to the saw of the sawfly, but resembling a needle. With
this she stings or punctures the surface of leaves, buds, stalks, or
even roots of various plants. In the wound thus produced she lays one or
more eggs. The effects of this proceeding, and particularly of the
irritating fluid which she injects into the wound, is to produce a
tumour or gall, within which the egg hatches, and on which the larva, a
thick fleshy grub (Plate II., Fig. 7), feeds. In some species each gall
contains a single larva; in others, several live together.

The oak supports several kinds of gallflies: one produces the well-known
oak-apple, one a small swelling on the leaf resembling a currant,
another a gall somewhat like an acorn, another attacks the root; the
species making the bullet-like galls, which are now so common, has only
existed for a few years in this country; the beautiful little spangles
so common in autumn on the under side of oak leaves are the work of
another species, the _Cynus longipennis_. One curious point about this
group is, that in some of the commonest species the females alone are
known, no one yet having ever succeeded in finding a male.

Another great family of the Hymenoptera is that of the ichneumons; the
females lay their eggs either in or on other insects, within the bodies
of which the larvae live. These larvae are thick, fleshy, legless grubs,
and feed on the fatty tissues of their hosts, but do not attack the
vital organs. When full-grown, the grubs eat their way through the skin
of the insect, and turn into chrysalides. Almost every kind of insect
is subject to the attacks of these little creatures, which are no doubt
useful in preventing the too great multiplication of insects, and
especially of caterpillars. Some species are so minute that they
actually lay their eggs within those of other insects (Figs. 15, 16).
These parasites assume very curious forms in their larval state.

But of all the Hymenoptera, the group containing the ant, the bee, and
the wasp is the most interesting. This is especially the case with the
social species, though the solitary ones also are extremely remarkable.
The solitary bee or wasp, for instance, forms a cell generally in the
ground, places in it a sufficient amount of food, lays an egg, and
closes the cell. In the case of bees, the food consists of honey; in
that of wasps, the larva requires animal food, and the mother therefore
places a certain number of insects in the cell, each species having its
own special prey, some selecting small caterpillars, some beetles, some
spiders. _Cerceris bupresticida_, as its name denotes, attacks beetles
belonging to the genus _Buprestis_. Now if the Cerceris were to kill the
beetle before placing it in the cell, it would decay, and the young
larva, when hatched, would find only a mass of corruption. On the other
hand, if the beetle were buried uninjured, in its struggles to escape it
would be almost certain to destroy the egg. The wasp has, however, the
instinct of stinging its prey in the centre of the nervous system, thus
depriving it of motion, and let us hope of suffering, but not of life;
consequently, when the young larva leaves the egg, it finds ready a
sufficient store of wholesome food.

Other wasps are social, and, like the bees and ants, dwell together in
communities. They live for one season, dying in autumn, except some of
the females, which hibernate, awake in the spring, and form new
colonies. These, however, do not, under ordinary circumstances, live
through a second winter. One specimen which I kept tame through last
spring and summer, lived until the end of February, but then died. The
larvae of wasps (Plate II., Fig. 9) are fat, fleshy, legless grubs. When
full-grown they spin for themselves a silken covering, within which they
turn into chrysalides. The oval bodies which are so numerous in ants'
nests, and which are generally called ants' eggs, are really not eggs
but cocoons. Ants are very fond of the honey-dew which is formed by the
Aphides, and have been seen to tap the Aphides with their antennae, as if
to induce them to emit some of the sweet secretion. There is a species
of _Aphis_ which lives on the roots of grass, and some ants collect these
into their nests, keeping them, in fact, just as we do cows. Moreover
they collect the eggs in the autumn and tend them through the winter
(when they are of no use) with the same care as their own, so as to have
a supply of young Aphides in the spring. This is one of the most
remarkable facts I know in the whole history of animal life. One species
of red ant does no work for itself, but makes slaves of a black kind,
which then do everything for their masters. The slave makers will not
even put food into their own mouths, but would starve in the midst of
plenty, if they had not a slave to feed them. I found, however, that I
could keep them in life and health for months if I gave them a slave for
an hour or two in a week to clean and feed them.

Ants also keep a variety of beetles and other insects in their nests.
That they have some reason for this seems clear, because they readily
attack any unwelcome intruder; but what that reason is, we do not yet
know. If these insects are to be regarded as the domestic animals of the
ants, then we must admit that the ants possess more domestic animals
than we do.

Some indeed of these beetles produce a secretion which is licked by the
ants like the honey-dew; there are others, however, which have not yet
been shown to be of any use to the ants, and yet are rarely, if ever,
found, excepting in ants' nests.

M. Lespes, who regards these insects as true domestic animals, has
recorded[8] some interesting observations on the relations between one
of them (_Claviger Duvalii_) and the ants (_Lasius niger_) with which it
lives. This species of _Claviger_ is never met with except in ants'
nests, though on the other hand there are many communities of _Lasius_
which possess none of these beetles; and M. Lespes found that when he
placed _Clavigers_ in a nest of ants which had none of their own, the
beetles were immediately killed and eaten, the ants themselves being on
the other hand kindly received by other communities of the same species.
He concludes from these observations that some communities of ants are
more advanced in civilization than others; the suggestion is no doubt
ingenious, and the fact curiously resembles the experience of navigators
who have endeavoured to introduce domestic animals among barbarous
tribes; but M. Lespes has not yet, so far as I am aware, published the
details of his observations, without which it is impossible to form a
decided opinion. I have sometimes wondered whether the ants have any
feeling of reverence for these beetles; but the whole subject is as yet
very obscure, and would well repay careful study.

[Illustration: PLATE III.--MATURE INSECTS.

Fig. 1, _Chloeon_; 2, _Meloe_ (after Shuckard); 3, _Calepteryx_;
4, _Sitaris_ (after Shuckard); 5, _Campodea_ (after Gervais);
6, _Acilius_; 7, _Termes_; 8, _Stylops_ (female); 9, _Thrips_.]

[Illustration: PLATE IV. YOUNG FORMS OF THE INSECTS REPRESENTED ON PLATE
III.--Fig. 1, Larva of _Chloeon_; 2, Larva of _Meloe_ (after Chapuis and
Candeze); 3, Larva of _Calepteryx_ (after Leon Dufour); 4, Larva of
_Sitaris_; 5, Larva of _Campodea_; 6, Larva of _Acilius_;
7, Larva of Termes (after Blanchard); 8, Larva of _Stylops_; 9, Larva
of _Thrips_.]

The order Strepsiptera are a small, but very remarkable group of
insects, parasitic on bees and wasps. The larva (Pl. IV., Fig. 8) is
minute, six-legged, and very active; it passes through its
transformations within the body of the bee or wasp. The male and female
are very dissimilar. The males are minute, very active, short-lived, and
excitable, with one pair of large membranous wings. The females (Pl.
III., Fig. 8), on the contrary, are almost motionless, and shaped very
much like a bottle; they never quit the body of the bee, but only thrust
out the top of the bottle between the abdominal rings of the bee.

In the order Coleoptera, the larvae differ very much in form. The
majority are elongated, active, hexapod, and more or less depressed; but
those of the Weevils (Pl. II., Fig. 6), of _Scolytus_ (Pl. II., Fig. 4),
&c., which are vegetable feeders, and live surrounded by their
food,--as, for instance, in grain, nuts, &c.,--are apod, white, fleshy
grubs, not unlike those of bees and ants. The larvae of the Longicorns,
which live inside trees, are long, soft, and fleshy, with six short
legs. The Geodephaga, corresponding with the Linnaean genera _Cicindela_
and _Carabus_, have six-legged, slender, carnivorous larvae; those of
_Cicindela_, which waylay their prey, being less active than the hunting
larvae of the Carabidae. The Hydradephaga, or water-beetles (Dyticidae and
Gyrinidae), have long and narrow larvae (Pl. IV., Fig. 6), with strong
sickle-shaped jaws, short antennae, four palpi, and six small eyes on
each side of the head; they are very voracious. The larvae of the
Staphylinidae are by no means unlike the perfect insect, and are found in
similar situations; their jaws are powerful, and their legs moderately
strong. The larvae of the Lamellicorn beetles (Figs. 1-6)--cockchafers,
stag-beetles, &c.--feed on vegetable substances or on dead animal
matter. They are long, soft, fleshy grubs, with the abdomen somewhat
curved, and generally lie on their side. The larvae of the Elateridae,
known as wireworms, are long and slender, with short legs. That of the
glowworm (Lampyridae) is not unlike the apterous female. The male
glowworm, on the contrary, is very different. It has long, thin, brown
wing-cases, and often flies into rooms at night, attracted by the light,
which it probably mistakes for that of its mate.

The metamorphoses of the Cantharidae are very remarkable, and will be
described subsequently. The larvae are active and hexapod. The Phytophaga
(_Crioceris_, _Galeruca_, _Haltica_, _Chrysomela_, &c.) are vegetable
feeders, both as larvae and in the perfect state. The larvae are furnished
with legs, and are not unlike the caterpillars of certain Lepidoptera.

The larva of _Coccinella_ (the Ladybird) is somewhat depressed, of an
elongated ovate form, with a small head, and moderately strong legs. It
feeds on Aphides.

Thus, then, we see that there are among the Coleoptera many different
forms of larvae. Macleay considered that there were five principal types.

1. Carnivorous hexapod larvae, with an elongated, more or less flattened
body, six eyes on each side of the head, and sharp falciform mandibles
(_Carabus_, _Dyticus_, &c.).

2. Herbivorous hexapod larvae, with fleshy, cylindrical bodies, somewhat
curved, so that they lie on their side.

3. Apod grub-like larvae, with scarcely the rudiments of antennae
(_Curculio_).

4. Hexapod antenniferous larvae, with a subovate body, the second segment
being somewhat larger than the others (_Chrysomela_, _Coccinella_).

5. Hexapod antenniferous larvae, of oblong form, somewhat resembling the
former, but with caudal appendages (_Meloe_, _Sitaris_).

The pupa of the Coleoptera is quiescent, and "the parts of the future
beetle are plainly perceivable, being encased in distinct sheaths; the
head is applied against the breast; the antennae lie along the sides of
the thorax; the elytra and wings are short and folded at the sides of
the body, meeting on the under side of the abdomen; the two anterior
pairs of legs are entirely exposed, but the hind pair are covered by
wing-cases, the extremity of the thigh only appearing beyond the sides
of the body."[9]

In the next three orders--namely, the Orthoptera (grasshoppers, locusts,
crickets, walking-stick insects, cockroaches, &c.), Euplexoptera
(earwigs), and Thysanoptera, a small group of insects well known to
gardeners under the name of _Thrips_ (Pl. I. and II., Figs. 1 and 2)--the
larvae when they quit the egg already much resemble the mature form,
differing, in fact, principally in the absence of wings, which are more
or less gradually acquired, as the insect increases in size. They are
active throughout life. Those specimens which have rudimentary wings
are, however, usually called pupae.

The Neuroptera present, perhaps, more differences in the character of
their metamorphoses than any other order of insects. Their larvae are
generally active, hexapod little creatures, and do not vary from one
another in appearance so much, for instance, as those of the Coleoptera,
but their pupae differ essentially; some groups, namely, the Psocidae,
Termitidae, Libellulidae, Ephemeridae, and Perlidae, remaining active
throughout life, like the Orthoptera; while a second division, including
the Myrmeleonidae, Hemerobiidae, Sialidae, Panorpidae, Raphidiidae, and
Mantispidae, have quiescent pupae, which, however, in some cases, acquire
more or less power of locomotion shortly before they assume the mature
state; thus that of _Raphidia_, though motionless at first, at length
acquires strength enough to walk, even while still enclosed in the pupa
skin, which is very thin.[10]

One of the most remarkable families belonging to this order is that of
the Termites, or white ants. They abound in the tropics, where they are
a perfect pest, and a serious impediment to human development. Their
colonies are extremely numerous, and they attack woodwork and furniture
of all kinds, generally working from within, so that their presence is
often unsuspected, until it is suddenly found that they have completely
eaten away the interior of some post or table, leaving nothing but a
thin outer shell. Their nests, which are made of earth, are sometimes
ten or twelve feet high, and strong enough to bear a man. One species,
_Termes lucifugus_, is found in the South of France, where it has been
carefully studied by Latreille. He found in these communities five kinds
of individuals--(1) males; (2) females, which grow to a very large size,
their bodies being distended with eggs, of which they sometimes lay as
many as 80,000 in a day; (3) a form described by some observers as Pupae,
but by others as neuters. These differ very much from the others, having
a long, soft body without wings, but with an immense head, and very
large, strong jaws. These individuals act as soldiers, doing apparently
no work, but keeping watch over the nest and attacking intruders with
great boldness. (4) Apterous, eyeless individuals, somewhat resembling
the winged ones, but with a larger and more rounded head; these
constitute the greater part of the community, and, like the workers of
ants and bees, perform all the labour, building the nest and collecting
food. (5) Latreille mentions another kind of individual which he regards
as the pupa, and which resembles the workers, but has four white
tubercles on the back, where the wings afterwards make their appearance.
There is still, however, much difference of opinion among entomologists,
with reference to the true nature of these different classes of
individuals. M. Lespes, who has recently studied the same species,
describes a second kind of male and a second kind of female, and the
subject, indeed, is one which offers a most promising field for future
study.

Another interesting family of Neuroptera is that of the Ephemerae, or
Mayflies (Pl. III., Fig. 1), so well known to fishermen. The larvae (Pl.
IV., Fig. 1) are semi-transparent, active, six-legged little creatures,
which live in water; having at first no gills, they respire through the
general surface of the body. They grow rapidly and change their skin
every few days. After one or two moults they acquire seven pairs of
branchiae, or gills, which are generally in the form of leaves, one pair
to the segment. When the larvae are about half grown, the posterior
angles of the two posterior thoracic segments begin to elongate. These
elongations become more and more marked with every change of skin. One
morning, in the month of June, some years ago, I observed a full-grown
larva, which had a glistening appearance, owing to the presence of a
film of air under the skin. I put it under the microscope, and, having
added a drop of water with a pipette, looked through the glass. To my
astonishment, the insect was gone, and an empty skin only remained. I
then caught a second specimen, in a similar condition, and put it under
the microscope, hoping to see it come out. Nor was I disappointed. Very
few moments had elapsed, when I had the satisfaction of seeing the
thorax open along the middle of the back; the two sides turned over; the
insect literally walked out of itself, unfolded its wings, and in an
instant flew up to the window. Several times since, I have had the
pleasure of witnessing this marvellous change, and it is really
wonderful how rapidly it takes place: from the moment when the skin
first cracks, not ten seconds are over before the insect has flown away.

Another family of Neuroptera, the Dragon-flies, or Horse-stingers, as
they are sometimes called, from a mistaken idea that they sting severely
enough to hurt a horse, though in fact they are quite harmless, also
spend their early days in the water. The larvae are brown, sluggish, ugly
creatures, with six legs. They feed on small water-animals, for which
they wait very patiently, either at the bottom of the water, or on some
aquatic plant. The lower jaws are attached to a long folding rod; and
when any unwary little creature approaches too near the larva, this
apparatus is shot out with such velocity that the prey which comes
within its reach seldom escapes. In their perfect condition, also,
Dragon-flies feed on other insects, and may often be seen hawking round
ponds. The so-called Ant-lions in many respects resemble the
Dragon-flies, but the habits of the larvae are very dissimilar. They do
not live in the water, but prefer dry places, where they bury themselves
in the loose sand, and seize with their long jaws any small insect which
may pass. The true Ant-lion makes itself a round, shallow pit in loose
ground or sand, and buries itself at the bottom. Any inattentive little
insect which steps over the edge of this pit immediately falls to the
bottom, and is instantaneously seized by the Ant-lion. Should the insect
escape, and attempt to climb up the side of the pit, the Ant-lion is
said to throw sand at it, knocking it down again.

One other family of Neuroptera which I must mention, is the
Hemerobiidae. The perfect insect is a beautiful, lace-winged, very
delicate, green creature, something like a tender Dragon-fly, and with
bright, green, touching eyes. The female deposits her eggs on leaves,
not directly on the plant itself, but attached to it by a long white
slender footstalk. The larva has six legs and powerful jaws, and makes
itself very useful in destroying the Hop-fly.

The insects forming the order Trichoptera are well known in their larval
condition, under the name of caddis worms. These larvae are not
altogether unlike caterpillars in form, but they live in water--which is
the case with very few lepidopterous larvae--and form for themselves
cylindrical cases or tubes, built up of sand, little stones, bits of
stick, leaves, or even shells. They generally feed on vegetable
substances, but will also attack minute freshwater animals. When full
grown, the larva fastens its case to a stone, the stem of a plant, or
some other fixed substance, and closes the two ends with an open grating
of silken threads, so as to admit the free access of water, while
excluding enemies. It then turns into a pupa which bears some
resemblance to the perfect insect, "except that the antennae, palpi,
wings, and legs are shorter, enclosed in separate sheaths, and arranged
upon the breast." The pupa remains quiet in the tube until nearly ready
to emerge, when it comes to the surface, and in some cases creeps out of
the water. It is not therefore so completely motionless as the pupae of
Lepidoptera.

The Diptera, or Flies, comprise insects with two wings only, the hinder
pair being represented by minute club-shaped organs called "halteres."
Flies quit the egg generally in the form of fat, fleshy, legless grubs.
They feed principally on decaying animal or vegetable matter, and are no
doubt useful as scavengers. Other species, as the gadflies, deposit
their eggs on the bodies of animals, within which the grubs feed, when
hatched. The mouth is generally furnished with two hooks which serve
instead of jaws. The pupae of Diptera are of two kinds. In the true
flies, the outer skin of the full-grown larva is not shed, but contracts
and hardens, thus assuming the appearance of an oval brownish shell or
case, within which the insect changes into a chrysalis. The pupae of the
gnats, on the contrary, have the limbs distinct and enclosed in sheaths.
They are generally inactive, but some of the aquatic species continue to
swim about.

One group of Flies, which is parasitic on horses, sheep, bats, and other
animals, has been called the Pupipara, because it was supposed that they
were not born until they had arrived at the condition of pupae. They come
into the world in the form of smooth, ovate bodies, much resembling
ordinary dipterous pupae, but as Leuckart has shown,[11] they are true,
though abnormal, larvae.

The next order, that of the Aphaniptera, is very small in number,
containing only the different species of Flea. The larva is long,
cylindrical, and legless; the chrysalis is motionless, and the perfect
insect is too well known, at least, as regards its habits, to need any
description.

The Heteroptera, unlike the preceding orders of insects, quit the egg in
a form differing from that of the perfect insect principally in the
absence of wings, which are gradually acquired. In their metamorphoses
they resemble the Orthoptera, and are active through life. The majority
are dull in colour, though some few are very beautiful. The species
constituting this group, though very numerous, are generally small, and
not so familiarly known to us as those of the other large orders, with
indeed one exception, the well-known Bug. This is not, apparently, an
indigenous insect, but seems to have been introduced. The word is indeed
used by old writers, but either as meaning a bugbear, or in a general
sense, and not with reference to this particular insect. In this country
it never acquires wings, but is stated to do so sometimes in warmer
climates. The Heteroptera cannot exactly be said either to sting or
bite. The jaws, of which, as usual among insects, there are two pairs,
are like needles, which are driven into the flesh, and the blood is then
sucked up the lower lip, which has the form of a tube. This peculiar
structure of the mouth prevails throughout the whole order; consequently
their nutriment consists almost entirely of the juices of animals or
plants. The Homoptera agree with the Heteroptera in the structure of the
mouth, and in the metamorphoses. They differ principally in the front
wings, which in Homoptera are membranous throughout, while in the
Heteroptera, the front part is thickened and leathery. As in the
Heteroptera, however, so also in the Homoptera, some species do not
acquire wings. The Cicada, celebrated for its chirp, and the lanthorn
fly, belong to this group. So also does the so-called Cuckoo-spit, so
common in our gardens, which has the curious faculty of secreting round
itself a quantity of frothy fluid which serves to protect it from its
enemies. But the best known insects of this group are the Aphides or
Plant-lice; while the most useful belong to the Coccidae, or scale
insects, from one species of which we obtain the substance called lac,
so extensively used in the manufacture of sealing-wax and varnish.
Several species also have been used in dyeing, especially the Cochineal
insect of Mexico, a species which lives on the cactus. The male _Coccus_
is a minute, active insect, with four large wings; while the female, on
the contrary, never acquires wings, but is very sluggish, broad, more or
less flattened, and in fact, when full grown, looks like a small brown,
red, or white scale.

The larva of the order Lepidoptera are familiar to us all, under the
name of caterpillars. The insects of this order in their larval
condition are almost all phytophagous, and are very uniform both in
structure and in habits. The body is long and cylindrical, consisting of
thirteen segments; the head is armed with powerful jaws; the three
following segments, the future prothorax, mesothorax, and meta-thorax,
each bears a pair of simple articulated legs. Of the posterior segments,
five also bear false or pro-legs, which are short, unjointed, and
provided with a number of hooklets. A caterpillar leads a dull and
uneventful life; it eats ravenously, and grows rapidly, casting its skin
several times during the process, which generally lasts only a few
weeks; though in some cases, as for instance that of the goat-moth, it
extends over a period of two or three years, after which the larva
changes into a quiescent pupa or chrysalis.




CHAPTER II.

_THE INFLUENCE OF EXTERNAL CONDITIONS ON THE FORM AND STRUCTURE OF
LARVAE._


The facts recapitulated briefly in the preceding chapter show, that the
forms of insect larvae depend greatly on the group to which they belong.
Thus the same tree may harbour larvae of Diptera, Hymenoptera,
Coleoptera, and Lepidoptera; each presenting the form typical of the
family to which it belongs.

If, again, we take a group, such, for instance, as the Lamellicorn
beetles, we shall find larvae extremely similar in form, yet very
different in habits. Those, for instance, of the common cockchafer (Fig.
1) feed on the roots of grass; those of _Cetonia aurata_ (Fig. 2)
inhabit ants' nests; the larvae of the genus _Trox_ (Fig. 3) are found on
dry animal substances; of _Oryctes_ (Fig. 4) in tan-pits; of _Aphodius_
(Fig. 5) in dung; of _Lucanus_ (the stag-beetle, Fig. 6) in wood.

[Illustration: FIG. 1, Larva of the Cockchafer (_Melolontha_).
(Westwood, Int. to the Modern Classification of Insects, vol. i. p.
194.). 2, Larva of _Cetonia_. 3, Larva of _Trox_. 4, Larva of _Oryctes_.
5, Larva of _Aphodius_ (Chapuis and Candeze, Mem. Soc. Roy. Liege,
1853). 6, Larva of _Lucanus_. (Packard, Guide to the Study of Insects,
Fig. 403).]

On the other hand, in the present chapter it will be my object to show
that the form of the larva depends very much on the conditions of its
life. Thus, those larvae which are internal parasites, whether in
animals or plants, are vermiform, as are those which live in cells, and
depend on their parents for food. On the other hand, larvae which burrow
in wood have strong jaws and generally somewhat weak thoracic legs;
whilst those which feed on leaves have the thoracic legs more developed,
but less so than the carnivorous species. Now, the Hymenoptera, as a
general rule, belong to the first category: the larvae of the Ichneumons,
&c., which live in animals,--those of the Cynipidae, inhabiting
galls,--and those of ants, bees, wasps, &c., which are fed by their
parents, are fleshy, apodal grubs; though the remarkable fact that the
embryos of bees in one stage of their development possess rudiments of
thoracic legs which subsequently disappear, seems to show, not indeed
that the larvae of bees were ever hexapod, but that bees are descended
from ancestors which had hexapod larvae, and that the present apod
condition of these larvae is not original, but results from their mode of
life.

On the other hand, the larvae of _Sirex_ (Fig. 14) being wood-burrowers,
possess well-developed thoracic legs. Again, the larvae of the
Tenthredinidae, which feed upon leaves, closely resemble the caterpillars
of Lepidoptera, even to the presence of abdominal pro-legs.

[Illustration: FIG. 7, Larva of _Brachytarsus_ (Ratzeburg, Forst.
Insecten). 8, Larva of _Crioceris_ (Westwood, loc. cit.).]

The larvae of most Coleoptera (Beetles) are active, hexapod, and more or
less flattened: but those which live inside vegetable tissues, such as
the weevils, are apod fleshy grubs, like those of Hymenoptera. Pl. II.,
Fig. 6, represents the larva of the nut-weevil, _Balaninus_ (Pl. I., Fig.
6), and it will be seen that it closely resembles Pl. II., Fig. 5, which
represents that of a fly (_Anthrax_), Pl. I., Fig. 5, and Pl. II., Figs.
7, 8, and 9, which represent respectively those of a _Cynips_ or gall-fly
(Pl. I., Fig. 7), an ant (Pl. I., Fig. 8), and wasp (Pl. I., Fig. 9).
Nor is _Balaninus_ the only genus of Coleoptera which affords us examples
of this fact. Thus in the genus _Scolytus_ (Pl. I., Fig. 4), the larvae
(Pl. II., Fig. 4), which, as already mentioned, feed on the bark of the
elm, closely resemble those just described, as also do those of
_Brachytarsus_ (Fig. 7). On the other hand, the larvae of certain beetles
feed on leaves, like the caterpillars of Lepidoptera; thus that of
_Crioceris Asparagi_ (Fig. 8)--which, as its name denotes, feeds on the
asparagus--closely resembles the larvae of certain Lepidoptera, as for
instance of _Thecla spini_. From this point of view the transformations
of the genus _Sitaris_ (Pl. III., Fig. 4), which have been very carefully
investigated by M. Fabre, are peculiarly interesting.[12]

[Illustration: FIG. 9, Larva of _Sitaris numeralis_ (Fabre, Ann. des Sci.
Nat., ser. 4, tome vii.). 10, Larva of _Sitaris humeralis_, in the second
stage. 11, Larva of _Sitaris humeralis_, in the third stage. 12, Larva of
_Sitaris humeralis_, in the fourth stage. 13, Pupa of _Sitaris_.]

The genus _Sitaris_ (a small beetle allied to Cantharis, the blister-fly,
and to _Meloe_, the oil-beetle) is parasitic on a kind of Bee
(Anthophora), which excavates subterranean galleries, each leading to a
cell. The eggs of the _Sitaris_, which are deposited at the entrance of
these galleries, are hatched at the end of September or beginning of
October; and M. Fabre not unnaturally expected that the young larvae,
which are active little creatures with six serviceable legs (Fig. 9),
would at once eat their way into the cells of the Anthophora. No such
thing: till the month of April following they remain without leaving
their birthplace, and consequently without food; nor do they in this
long time change either in form or size. M. Fabre ascertained this, not
only by examining the burrows of the _Anthophoras_, but also by direct
observation of some young larvae kept in captivity. In April, however,
his captives at last awoke from their long lethargy, and hurried
anxiously about their prisons. Naturally inferring that they were in
search of food, M. Fabre supposed that this would consist either of the
larvae or pupae of the Anthophora, or of the honey with which it stores
its cell. All three were tried without success. The first two were
neglected, and the larvae, when placed on the latter, either hurried
away, or perished in the attempt, being evidently unable to deal with
the sticky substance. M. Fabre was in despair: "Jamais experience," he
says, "n'a eprouve pareille deconfiture. Larves, nymphes, cellules,
miel, je vous ai tous offert; que voulez-vous donc, bestioles maudites?"
The first ray of light came to him from our countryman, Newport, who
ascertained that a small parasite found by Leon Dufour on one of the
wild bees, and named by him Triungulinus, was, in fact, the larva of
_Meloe_. The larvae of _Sitaris_ much resembled Dufour's Triungulinus;
and acting on this hint, M. Fabre examined many specimens of Anthophora,
and found on them at last the larvae of his _Sitaris_. The males of
Anthophora emerge from the pupae sooner than the females, and M. Fabre
ascertained that, as they come out of their galleries, the little
_Sitaris_ larvae fasten upon them. Not, however, for long: instinct
teaches them that they are not yet in the straight path of development;
and, watching their opportunity, they pass from the male to the female
bee. Guided by these indications, M. Fabre examined several cells of the
Anthophora: in some, the egg of the Anthophora floated by itself on the
surface of the honey; in others, on the egg, as on a raft, sat the still
more minute larva of the _Sitaris_. The mystery was solved. At the
moment when the egg is laid the _Sitaris_ larva springs upon it. Even
while the poor mother is carefully fastening up her cell, her mortal
enemy is beginning to devour her offspring: for the egg of the
Anthophora serves not only as a raft, but as a repast. The honey which
is enough for either, would be too little for both; and the _Sitaris_,
therefore, at its first meal, relieves itself from its only rival. After
eight days the egg is consumed, and on the empty shell the _Sitaris_
undergoes its first transformation, and makes its appearance in a very
different form, as shown in Fig. 10.

The honey which was fatal before is now necessary; the activity which
before was necessary is now useless; consequently, with the change of
skin, the active, slim larva changes into a white, fleshy grub, so
organized as to float on the surface of the honey, with the mouth
beneath, and the spiracles above the surface: "grace a l'embonpoint du
ventre," says M. Fabre, "la larve est a l'abri de l'asphyxie." In this
state it remains until the honey is consumed; then the animal
contracts, and detaches itself from its skin, within which the further
transformations take place. In the next stage, which M. Fabre calls the
pseudo-chrysalis (Fig. 11), the larva has a solid corneous envelope and
an oval shape; and in its colour, consistency, and immobility reminds
one of a Dipterous pupa. The time passed in this condition varies much.
When it has elapsed, the animal moults again, again changes its form,
and assumes that shown in Fig. 12; after this it becomes a pupa (Fig.
13) without any remarkable peculiarities. Finally, after these wonderful
changes and adventures, in the month of August the perfect _Sitaris_ (Pl.
III., Fig. 4) makes its appearance.

On the other hand, there are cases in which larvae diverge remarkably
from the ordinary type of the group to which they belong, without, as it
seems in our present imperfect state of information, any sufficient
reason.

Thus the ordinary type of Hymenopterous larva, as we have already seen,
is a fleshy apod grub; although those of the leaf-eating and wood-boring
groups, Tenthredinidae and Siricidae (Fig. 14), are caterpillars, more or
less closely resembling those of Lepidoptera. There is, however, a group
of minute Hymenoptera, the larvae of which reside within the eggs or
larvae of other insects. It is difficult to understand why these larvae
should differ from those of Ichneumons, which are also parasitic
Hymenoptera, and should be, as will be seen by the accompanying figures,
of such remarkable and grotesque forms. The first known of these curious
larvae was observed by De Filippi,[13] who, having collected some of the
transparent eggs of a small Beetle (_Rhynchites betuleti_), to his great
surprise found more than half of them attacked by a parasite, which
proved to be the larva of a minute Hymenopterous insect belonging to the
Pteromalidae. Fig. 15 shows the egg of the Beetle, with the parasitic
larva, which is represented on a larger scale in Fig. 16.

[Illustration: FIG. 14, Larva of _Sirex_ (Westwood, loc. cit.). 15, Egg of
_Rhynchites_, showing the parasitic Larva in the interior. 16, the
parasitic Larva more magnified.]

More recently this group has been studied by M. Ganin,[14] who thus
describes the development of _Platygaster_. The egg, as in allied
Hymenopterous families, for instance in _Cynips_, is elongated and
club-shaped (Fig. 17). After a while a large nucleated cell appears in
the centre (Fig. 18). This nucleated cell divides (Fig. 19) and
subdivides. The outermost cells continue the same process, thus forming
an outer investing layer. The central, on the contrary, enlarges
considerably, and develops within itself a number of daughter cells
(Figs. 20 and 21), which gradually form a mulberry-like mass, thus
giving rise to the embryo (Fig. 22).

[Illustration: FIG. 17, Egg of _Platygaster_ (after Ganin). 18, Egg of
_Platygaster_ showing the central cell. 19, Egg of _Platygaster_ after the
division of the central wall. 20, Egg of Platygaster more advanced. 21,
Egg of _Platygaster_ more advanced. 22, Egg of Platygaster showing the
rudiment of the embryo.]

Ganin met with the larvae of _Platygaster_ in those of a small gnat,
_Cecidomyia_. Sometimes as many as fifteen parasites occurred in one gnat,
but as a rule only one of these attained maturity. The three species of
_Platygaster _differ considerably in form, as shown in Figs. 23-25. They
creep about within the larva of _Cecidomyia_ by means of the strong hooked
feet, _kf_, somewhat aided by movements of the tail. They possess a
mouth, stomach, and muscles, but the nervous, vascular, and respiratory
systems do not make their appearance until later. After some time the
larva (Fig. 23) changes its skin, assuming the form represented in Fig.
26. In this moult the last abdominal segment of the first larva is
entirely thrown off: not merely the outer skin, as in the case of the
other segments, but also the hypodermis and the muscles. This larva, as
will be seen by the figure, resembles a barrel or egg in form, and is
.870 mm. in length, the external appendages having disappeared, and the
segments being indicated only by the arrangement of the muscles. _slkf_
is the oesophagus leading into a wide stomach which occupies nearly
the whole body, _gsae_ is the rudiment of the supra-oesophageal
ganglia, _bsm_ the ventral nervous cords. The ventral nervous mass has
the form of a broad band, with straight sides; it consists of embryonal
cells, and remains in this undeveloped condition during the whole larval
state.

[Illustration: FIG. 23, Larva of _Platygaster_ (after Ganin)--_mo_, mouth;
_a_, antenna; _kf_, hooked feet; _z_, toothed process; _lfg_, lateral
process; _f_, branches of the tail. 24, Larva of another species of
_Platygaster_. The letters indicate the same parts as in the preceding
figure. 25, Larva of a third species of _Platygaster_. The letters
indicate the same parts as in the preceding figures. 26, Larva of
_Platygaster_ in the second stage--_mo_, mouth; _slkf_, oesophagus;
_gsae_, supra-oesophageal ganglion; _lm_, muscles; _bsm_, nervous
system; _ga_, _gh_, rudiments of the reproductive glands. 27, Larva of
_Platygaster_ in the third stage--_mo_, mouth; _md_, mandibles; _gsae_,
supra-oesophageal ganglion; _slk_, oesophagus; _ag_, ducts of the
salivary glands; _bnm_, ventral nervous system; _sp_, salivary glands;
_msl_, stomach; _im_, imaginal discs; _tr_, tracheae; _fk_, fatty tissue;
_ed_, intestine; _ga_, rudiments of reproductive organs; _ew_, wider
portion of intestine; _ao_, posterior opening.]

At the next moult the larva enters its third state, which, as far as the
external form (Fig. 27) is concerned, differs from the second only in
being somewhat more elongated. The internal organs, however, are much
more complex and complete. The tracheae have made their appearance, and
the mouth is provided with a pair of mandibles. From this point the
metamorphoses of _Platygaster_ do not appear to differ materially from
those of other parasitic Hymenoptera.

An allied genus, _Polynema_, has also very curious larvae. The perfect
insect is aquatic in its habits, swimming by means of its wings; flying,
if we may say so, under water.[15] It lays its eggs inside those of
Dragon-flies; and the embryo, as shown in Fig. 28, has the form of a
bottle-shaped mass of undifferentiated embryonal cells, covered by a
thin cuticle, but without any trace of further organization. Protected
by the egg-shell of the Dragon-fly, and bathed in the nourishing fluid
of the Dragon-fly's egg, the young _Polynema_ imbibes nourishment through
its whole surface, and increases rapidly in size. The digestive canal
gradually makes its appearance; the cellular mass forms a new skin
beneath the original cuticle, distinctly divided into segments, and
provided with certain appendages. After a while the old cuticle is
thrown off, and the larva gradually assumes the form shown in Fig. 29.
The subsequent metamorphoses of _Polynema_ offer no special peculiarities.

[Illustration: FIG. 28, Embryo of _Polynema_ (after Ganin). 29, Larva of
_Polynema_--_asch_, rudiments of the antenna; _flsch_, rudiments of the
wings; _bsch_, rudiments of the legs; _vfg_. lateral projections;
_gsch_, rudiments of the ovipositor; _fk_, fatty tissue.]

From these facts--and, if necessary, many more of the same nature might
have been brought forward--it seems to me evident that while the form of
any given larva depends to a certain extent on the group of insects to
which it belongs, it is also greatly influenced by the external
conditions to which it is subjected; that it is a function of the life
which the larva leads and of the group to which it belongs.

The larvae of insects are generally regarded as being nothing more than
immature states--as stages in the development of the egg into the
imago; and this might more especially appear to be the case with those
insects in which the larvae offer a general resemblance in form and
structure (excepting of course so far as relates to the wings) to the
perfect insect. Nevertheless we see that this would be a very incomplete
view of the case. The larva and pupa undergo changes which have no
relation to the form which the insect will ultimately assume. With a
general tendency to this goal, as regards size and the development of
the wings, there are coincident other changes having reference only to
existing wants and condition. Nor is there in this, I think, anything
which need surprise us. External circumstances act on the insect in its
preparatory states, as well as in its perfect condition. Those who
believe that animals are susceptible of great, though gradual, change
through the influence of external conditions, whether acting, as Mr.
Darwin has suggested, through natural selection, or in any other manner,
will see no reason why these changes should be confined to the mature
animal. And it is evident that creatures which, like the majority of
insects, live during the successive periods of their existence in very
different circumstances, may undergo considerable changes in their
larval organization, in consequence of forces acting on them while in
that condition; not, indeed, without affecting, but certainly without
affecting to any corresponding extent, their ultimate form.

I conclude, therefore, that the form of the larva in insects, whenever
it departs from the hexapod _Campodea_ type, has been modified by the
conditions under which it lives. The external forces acting upon it are
different from those which affect the mature form; and thus changes are
produced in the young which have reference to its immediate wants,
rather than to its final form.

And, lastly, as a consequence, that metamorphoses may be divided into
two kinds, developmental and adaptional or adaptive.




CHAPTER III.

_ON THE NATURE OF METAMORPHOSES._


In the preceding chapters we have considered the life history of insects
after they have quitted the egg; but it is obvious that to treat the
subject in a satisfactory manner we must take the development as a
whole, from the commencement of the changes in the egg, up to the
maturity of the animal, and not suffer ourselves to be confused by the
fact that insects leave the egg in very different stages of embryonal
development. For though all young insects when they quit the egg are
termed "larvae," whatever their form may be (the case of the so-called
Pupipara not constituting a true exception), still it must be remembered
that some of these larvae are much more advanced than others. It is
evident that the larva of a fly, as regards its stage of development,
corresponds in reality neither with that of a moth nor with that of a
grasshopper. The maggots of flies, in which the appendages of the head
are rudimentary, belong to a lower grade than the grubs of bees, &c.,
which have antennae, mandibles, maxillae, labrum, labium, and, in fact,
all the mouth parts of a perfect insect.

The caterpillars of Lepidoptera are generally classed with the vermiform
larva of Diptera and Hymenoptera, and contrasted with those of
Orthoptera, Hemiptera, &c.; but, in truth, the possession of thoracic
legs places them, together with the similar larvae of the Tenthredinidae,
on a decidedly higher level. Thus, then, the period of growth (that in
which the animal eats and increases in size) occupies sometimes one
stage in the development of an insect, sometimes another; sometimes, as
for instance in the case of _Chloeon_, it continues through more than
one; or, in other words, growth is accompanied by development. But, in
fact, the question is even more complicated than this. It is not only
that the larvae of insects at their birth offer the most various grades
of development, from the grub of a fly to the young of a grasshopper or
a cricket; but that, if we were to classify larvae according to their
development, we should have to deal, not with a simple case of
gradations only, but with a series of gradations, which would be
different according to the organ which we took as our test.

Apart, however, from the adaptive changes to which special reference was
made in the previous chapter, the differences which larvae present are
those of gradation, not of direction. The development of a grasshopper
does not pursue a different course from that of a butterfly, but the
embryo attains a higher state before quitting the egg in the former than
in the latter: while in most Hymenoptera, as for instance in Bees,
Wasps, Ants, &c., the young are hatched without thoracic appendages; in
the Orthoptera, on the contrary, the legs are fully developed before the
young animal quits the egg.

Prof. Owen,[16] indeed, goes so far as to say that the Orthoptera and
other Homomorphous insects are, "at one stage of their development,
apodal and acephalous larvae, like the maggot of the fly; but instead of
quitting the egg in this stage, they are quickly transformed into
another, in which the head and rudimental thoracic feet are developed to
the degree which characterizes the hexapod larvae of the _Carabi_ and
_Petalocera_."

[Illustration: FIG. 30, Egg of _Phryganea_ (Mystacides)--_A_^1,
mandibular segment; _C_^1 to _C_^5, maxillary, labial, and three
thoracic segments; _D_, abdomen (after Zaddach). 31, Egg of _Phryganea_
somewhat more advanced--_b_, mandibles; _c_, maxillae; _cfs_, rudiments
of the three pairs of legs.]

I quite believe that this may have been true of such larvae at an early
geological period, but the fact now appears to be, so far at least as
can be judged from the observations yet recorded, that the legs of those
larvae which leave the egg with these appendages generally make their
appearance before the body-walls have closed, or the internal organs
have approached to completion. Indeed, when the legs first appear, they
are merely short projections, which it is not always easy to distinguish
from the segments themselves. It must, however, be admitted, that the
observations are neither so numerous, nor in most cases so full, as
could be wished.

Fig. 30 represents an egg of a May-fly (_Phryganea_), as represented by
Zaddach in his excellent memoir,[17] just before the appearance of the
appendages. It will be seen that a great part of the yolk is still
undifferentiated, that the side walls are incomplete, the back quite
open, and the segments merely indicated by undulations. This stage is
rapidly passed through, and Zaddach only once met with an egg in this
condition; in every other specimen which had indications of segments,
the rudiments of the legs had also made their appearance, as in Fig. 31,
which, however, as will be seen, does not in other respects show much
advance on Fig. 30.

Again in _Aphis_, the embryology of which has been so well worked out by
Huxley,[18] the case is very similar, although the legs are somewhat
later in making their appearance. When the young was 1/140th of an inch
in length, he found the cephalic portion of the embryo beginning, he
says, "to extend upwards again over the anterior face of the germ, so as
to constitute its anterior and a small part of its superior wall. This
portion is divided by a median fissure into two lobes, which play an
important part in the development of the head, and will be termed the
'procephalic lobes.' I have already made use of this term for the
corresponding parts in the embryos of Crustacea. The rudimentary thorax
presents traces of a division into three segments; and the dorso-lateral
margins of the cephalic blastoderm, behind the procephalic lobes, have a
sinuous margin. It is in embryos between this and 1/100th of an inch in
length, that the rudiments of the appendages make their appearance; and
by the growth of the cephalic, thoracic, and abdominal blastoderm,
curious changes are effected in the relative position of those regions."

In _Chrysopa oculata_, one of the Hemerobiidae, Packard has described[19]
and figured a stage in which the body segments have made their
appearance, but in which he says "there are no indications of limbs. The
primitive band is fully formed, the protozorites being distinctly
marked, the transverse impressed lines indicating the primitive segments
being distinct, and the median furrow easily discerned." Here also,
again, the dorsal walls are incomplete, and the internal organs as yet
unformed.

In certain Dragon-flies (_Calepteryx_), and _Hemiptera_ (_Hydrometra_), the
legs, according to Brandt,[20] appear at a still earlier stage.

According to the observations of Koelliker,[21] it would appear that in
the Coleopterous genus _Donacia_ the segments and appendages appear
simultaneously.

Koelliker himself, however, frankly admits that "meae de hoc insecto
observationes satis sunt manca," and it is possible that he may never
have met with an embryo in the state immediately preceding the
appearance of the legs; especially as it appears from the observations
of Kowalevski that in _Hydrophilus_ the appendages do not make their
appearance until after the segments.[22]

On the whole, as far as we can judge from the observations as yet
recorded, it seems that in Homomorphous insects the ventral wall is
developed and divided into segments, before the appearance of the legs;
but that the latter are formed almost simultaneously with the cephalic
appendages, and before either the dorsal walls of the body or the
internal organs.

[Illustration: FIG. 32.--Egg of _Pholcus opilionides_ (after Claparede).]

As it is interesting, from this point of view, to compare the
development of other Articulata with that of insects, I give a figure
(Fig. 32), representing an early stage in the development of a spider
(_Pholcus_) after Claparede,[23] who says, "C'est a ce moment qu'a lieu
la formation des _protozonites_ ou segments primordiaux du corps de
l'embryon. Le rudiment ventral s'epaissit suivant six zones disposees
transversalement entre le capuchon anal et le capuchon cephalique."

[Illustration: FIG. 33.--Embryo of _Julus_ (after Newport).]

Among Centipedes the development of _Julus_ has been described by
Newport.[24] The first period, from the deposition of the egg to the
gradual bursting of the shell, and exposure of the embryo within it,
which, however, remains for some time longer in connection with the
shell, lasts for twenty-five days. The segments of the body, originally
six in number, make their appearance on the twentieth day after the
deposition of the egg, at which time there were no traces of legs. The
larva, when it leaves the egg, is a soft, white, legless grub (Fig. 33),
consisting of a head and seven segments, the head being somewhat firmer
in texture than the rest of the body. It exhibits rudimentary antennae,
but the legs are still only represented by very slight papilliform
processes on the undersides of the segments to which they belong.

As already mentioned, it is possible that at one time the vermiform
state of the Homomorphous insects--which, as we have seen, is now so
short, and passed through at so early a stage of development--was more
important, more prolonged, and accompanied by a more complete condition
of the internal organs. The compression, and even disappearance of those
embryonal stages which are no longer adapted to the mode of life--which
do not benefit the animal--is a phenomenon not without a parallel in
other parts of the animal or even of the vegetable kingdom. Just as in
language long compound words have a tendency to concision, and single
letters sometimes linger on, indicating the history of a word, like the
"l" in "alms," or the "b" in "debt," long after they have ceased to
influence the sound; so in embryology useless stages, interesting as
illustrations of past history, but without direct advantage under
present conditions, are rapidly passed through, and even, as it would
appear, in some cases altogether omitted.

[Illustration: FIG. 34.--Colony of _Bougainvillea fruticosa_, natural
size, to the underside of a piece of floating timber (after Allman).]

For instance, among the Hydroida, in the great majority of cases, the
egg produces a body more or less resembling the common _Hydra_ of our
ponds, and known technically as the "trophosome," which develops into
the well-known Medusae or jelly-fishes. The group, however, for which
Prof. Allman has proposed the term Monopsea,[25] and of which the genus
_Aegina_ may be taken as the type, is, as he says, distinguished by the
absence of a hydriform stage, "the ovum becoming developed through
direct metamorphosis into a medusiform body, just as in the other orders
it is developed into a hydriform body." Fig. 34 represents, after
Allman, a colony of _Bougainvillea fruticosa_ of the natural size. It is
a British species, which is found growing on buoys, floating timber,
&c., and, says Allman,[26] "when in health and vigour, offers a
spectacle unsurpassed in interest by any other species--every branchlet
crowned by its graceful hydranth and budding with Medusae in all stages
of development (Fig. 35), some still in the condition of minute buds, in
which no trace of the definite Medusa-form can yet be detected; others,
in which the outlines of the Medusa can be distinctly traced within the
transparent _ectotheque_ (external layer); others, again, just casting
off this thin outer pellicle, and others completely freed from it,
struggling with convulsive efforts to break loose from the colony, and
finally launched forth in the full enjoyment of their freedom into the
surrounding water. I know of no form in which so many of the
characteristic features of a typical hydroid are more finely expressed
than in this beautiful species."

[Illustration: FIG. 35.--Portion of colony of _Bougainvillea fruticosa_,
more magnified.]

[Illustration: FIG. 36.--The Medusa form of the same species.]

Fig. 36 represents the Medusa form of this species, and the development
thus described may be regarded as typical of the Hydroida; yet, as
already mentioned, the Aeginidae do not present us with any stage
corresponding to the fixed condition of _Bougainvillea_, but, on the
contrary, are developed into Medusae direct from the egg.

On the other hand, there are groups in which the Medusiform stage
becomes less and less important.

[Illustration: FIG. 37, Larva of Prawn, Nauplius stage (after F.
Mueller). 38, Larva of Prawn, more advanced, Zoea stage.]

The great majority of the higher Crustacea go through well-marked
metamorphoses. Figs. 37 and 38 represent two stages in the development
of the prawn. In the first (Fig. 37), representing the young animal as
it quits the egg, the body is more or less oval and unsegmented; there
is a median frontal eye, and three pairs of natatory feet, the first
pair simple, while the two posterior are two-branched. Very similar
larvae occur in various other groups of Crustacea. They were at first
regarded as mature forms, and O. F. Mueller gave them the name of
Nauplius. So also, the second or Zoea form (Fig. 38) was at first
supposed to be a mature animal, until its true nature was discovered by
Vaughan Thompson.

The Zoea form of larva differs from the perfect prawn or crab in the
absence of the middle portion of the body and its appendages. The
mandibles have no palpi, the maxillipeds or foot-jaws are used as feet,
whereas in the mature form they serve as jaws. Branchiae are either
wanting or rudimentary, respiration being principally effected through
the walls of the carapace. The abdomen and tail are destitute of
articulate appendages. The development of Zoea into the perfect animal
has been well described by Mr. Spence Bate[27] in the case of the common
crab (_Carcinus maenas_).

All crabs, as far as we know, with the exception of a species of land
crab (_Gegarcinus_), described by Westwood, pass through a stage more or
less resembling that shown in Fig. 38. On the other hand, the great
group of Edriopthalma, comprising Amphipoda (shore-hoppers, &c.) and
Isopoda (wood-lice, &c.) pass through no such metamorphosis; the
development is direct, as in the Orthoptera. It is true that one
species, _Tanais Dulongii_, though a typical Isopod in form and general
character, is said to retain in some points, and especially in the mode
of respiration, some peculiarities of the Zoea type; but this is quite
an exceptional case. In _Mysis_, says F. Mueller,[28] "there is still a
trace of the Nauplius stage; being transferred back to a period when it
had not to provide for itself, the Nauplius has become degraded into a
mere skin; in _Ligia_ this larva-skin has lost the traces of limbs, and
in _Philoscia_ it is scarcely demonstrable."

The Echinodermata in most cases "go through a very well-marked
metamorphosis, which often has more than one larval stage.... The mass
of more or less differentiated sarcode, of which the larva, or
pseud-embryo, as opposed to the Echinoderm within it, is made up, always
carries upon its exterior certain bilaterally-arranged ciliated bands,
by the action of which the whole organism is moved from place to place;
and it may be strengthened by the super-addition to it of a framework
of calcareous rods."[29] Mueller considered that the mouth and pharynx of
the larva were either absorbed or cast off with the calcareous rods, but
were never converted into the corresponding organs of the perfect
Echinoderm. According to A. Agassiz, however, this is not the case, but
on the contrary "the whole larva and all its appendages are gradually
drawn into the body, and appropriated."[30]

Fig. 39 represents the larva of a sea-egg (_Echino cidaris_) after
Mueller.[31] The body is transparent, shaped somewhat like a double
easel, but with two long horns in front, which, as well as the posterior
processes, are supported by calcareous rods. This larva swims by means
of minute vibratile hairs, or ciliae. It has a mouth, stomach, and in
fact a well-defined alimentary canal; but no nerves or other internal
organs have yet been discovered in it. After swimming about in this
condition for a while, it begins to show signs of change. An involution
of the integument takes place on one side of the back, and continues to
deepen till it reaches a mass or store of what is called blastema, or
the raw material of the animal body. This blastema then begins to
change, and gradually assumes the form of the perfect Echinoderm.[32]

[Illustration: FIG. 39.--Larva of _Echino cidaris_, seen from above x 6/10
(after Mueller).]

[Illustration: FIG. 40, Larva of _Echinus_, x 100. _A_, front arm; _F_,
arms of the mouth process; _B_, posterior side arm; _E_(1), accessory
arm of the mouth process; _a_, mouth; _a'_, oesophagus; _b_, stomach;
_b'_, intestine; _o_, posterior orifice; _d_, ciliated bands; _f_,
ciliated epaulets; _c_, disc of future _Echinus_ (after Mueller).]

Fig. 40 represents a larva, probably of another sea-egg (_Echinus
lividus_), from the Mediterranean, and shows the commencement of the
sea-egg within the body of the larva. The capital letters denote the
different arms: _a_ is the mouth, _a'_ the oesophagus, _b_ the
stomach, _b'_ the intestine, _f_ the ciliated lobes or epaulets, _c_ the
young sea-egg.

The development of the beautiful _Comatula rosacea_ (Fig. 41) has been
described in the "Philosophical Transactions," by Prof. Wyville Thomson
and Dr. Carpenter.[33] The larva quits the egg, as shown in Fig. 42, in
the form of an oval body about 1/30 inch in length, something like a
barrel, surrounded by four bands or hoops of long vibratile hairs or
ciliae. There is also a tuft of still longer hairs at the narrower
posterior end of the body. Gradually a number of minute calcareous
spines and plates make their appearance (Fig. 43) in the body of this
larva, and at length arrange themselves in a definite order, so as to
form a bent calcareous club or rod with an enlarged head.

[Illustration: FIG. 41.--_Comatula rosacea_ (after Forbes).]

[Illustration: FIG. 42, Larva of Comatula rosacea (after Thomson). 43,
Larva of _Comatula rosacea_, more advanced. 44, Larva of Comatula rosacea,
in the Pentacrinus state.]

As this process continues, the little creature gradually loses its power
of swimming, and, sinking to the bottom, looses the bands of ciliae, and
attaches itself by its base to some stone or other solid substance, the
knob of the club being free. The calcareous framework increases in size,
and the expanded head forms itself into a cup, round which from five to
fifteen delicate tentacles, as shown in Fig. 44, make their appearance.

In this stage the young animal resembles one of the stalked Crinoids, a
family of Echinoderms very abundant in earlier geological periods, but
which has almost disappeared, being, as we see, now represented by the
young states of existing more advanced, free, species. This attached,
plant-like condition of _Comatula_ was indeed at first supposed to be a
mature form, and was named Pentacrinus; but we now know that it is only
a stage in the development of _Comatula_. The so-called Pentacrinus
increases considerably in size, and after various gradual changes, which
time does not now permit me to describe, quits the stalk, and becomes a
free _Comatula_.

The metamorphoses of the Starfishes are also very remarkable. Sars
discovered, in the year 1835, a curious little creature about an inch in
length, which he named _Bipinnaria asterigera_ (Figs. 45-47), and which
he then supposed to be allied to the ciliograde Medusae. Subsequent
observations, however, made in 1844, suggested to him that it was the
larva of a Starfish, and in 1847 MM. Koren and Danielssen satisfied
themselves that this was the case.

Figs. 45 and 46 represent the front and side view of a Bipinnaria found
by Mueller[34] near Marseilles. _a_ is the mouth, _b_ the oesophagus,
_c_ the stomach, _c_' the intestine. Fig. 47 represents a somewhat older
specimen, in which the Starfish (_k_) is already beginning to make its
appearance.

[Illustration: FIG. 45, Larva of Starfish (Bipinnaria), x 100 (after
Mueller). 46, Larva of Starfish (Bipinnaria), x 100, seen from the
side--_a_, mouth; _b_, oesophagus; _c_, stomach; _c'_, intestine. 47,
Larva of another Bipinnaria, showing the commencement of the
Starfish--_g_, canal of the ciliated sac; _i_, rudiments of tentacles;
_d_, ciliated band.]

But while certain Starfishes thus go through metamorphoses similar in
character, and not less remarkable than those of sea-eggs, there are
others--as, for instance, the genus _Asteracanthion_--in which development
may be said to be direct--the organs and appendages special to the
Pseud-embryo being in abeyance; while in another genus, _Pteraster_, they
are reduced to a mere investing membrane.[35]

Among the Ophiurans also we find two well-marked types of development.
Some passing through metamorphoses, while others, as for instance
_Ophiopholis bellis_, "is developed very much after the method of
_Asteracanthion Muelleri_, without passing through the Plutean
stage."[36]

Even in the same species of Echinoderm the degree of development
attained by the larva differs to a certain extent according to the
temperature, the supply of food, &c. Thus in _Comatula_, specimens which
are liberally supplied with sea-water, and kept warm, hurry as it were
through their early stages, and the free larva becomes distorted by the
growing Pentacrinus (see Fig. 43), almost before it has attained its
perfect form. On the other hand, under less favourable conditions, if
the temperature is low and food less abundant, the early stages are
prolonged, the larva is longer lived, and reaches a much higher degree
of independent development. Similar differences occur in the development
of other animals, as for instance, in the Hydroids,[37] and among the
insects themselves, in Flies;[38] and it is obvious that these facts
throw much light on the nature and origin of the metamorphoses of
insects, which subject we shall now proceed to consider.




CHAPTER IV.

_ON THE ORIGIN OF METAMORPHOSES._


The question still remains, Why do insects pass through metamorphoses?
Messrs. Kirby and Spence tell us they "can only answer that such is the
will of the Creator;"[39] this, however, is a general confession of
faith, not an explanation of metamorphoses. So indeed they themselves
appear to have felt; for they immediately proceed to make a suggestion.
"Yet one reason," they say, "for this conformation may be hazarded. A
very important part assigned to insects in the economy of nature, as we
shall hereafter show, is that of speedily removing superabundant and
decaying animal and vegetable matter. For such agents an insatiable
voracity is an indispensable qualification, and not less so unusual
powers of multiplication. But these faculties are in a great degree
incompatible; an insect occupied in the work of reproduction could not
continue its voracious feeding. Its life, therefore, after leaving the
egg, is divided into three stages."

But there are some insects--as, for instance, the Aphides--which
certainly are not among the least voracious, and which grow and breed at
the same time. There are also many scavengers among other groups of
animals--such, for instance, as the dog, the pig, and the vulture--which
undergo no metamorphosis.

It is certainly true that, as a general rule, growth and reproduction do
not occur together; and it follows, almost as a necessary consequence,
that in such cases the first must precede the second. But this has no
immediate connection with the occurrence of metamorphoses. The question
is not, why an insect does not generally begin to breed until it has
ceased to grow, but why, in attaining to its perfect form, it passes
through such remarkable changes; why these changes are so sudden and
apparently violent; and why they are so often closed by a state of
immobility--that of the chrysalis or pupa; for undoubtedly the quiescent
and death-like condition of the pupa is one of the most remarkable
phenomena of insect-metamorphoses.

In the first place, it must be observed that many animals which differ
considerably in their mature state, resemble one another more nearly
when young. Thus birds of the same genus, or of closely allied genera,
which, when mature, differ much in colour, are often very similarly
 when young. The young of the lion and the puma are often
striped, and the foetal Black whale has teeth, like its ally the Sperm
whale.

In fact, the great majority of animals do go through well-marked
metamorphoses, though in many cases they are passed through within the
egg, and thus do not come within the popular ken. "La larve," says,
Quatrefages, "n'est qu'un embryon a vie independante."[40] Those
naturalists who accept in any form the theory of evolution, consider
that "the embryonal state of each species reproduces more or less
completely the form and structure of its less modified progenitors."[41]
"Each organism," says Herbert Spencer,[42] "exhibits within a short
space of time a series of changes which, when supposed to occupy a
period indefinitely great, and to go on in various ways instead of one
way, give us a tolerably clear conception of organic evolution in
general."

The naturalists of the older school do not, as Darwin and Fritz Mueller
have already pointed out, dispute these facts, though they explain them
in a different manner--generally by the existence of a supposed tendency
to diverge from an original type. Thus Johannes Mueller says, "The idea
of development is not that of mere increase of size, but that of
progress from what is not yet distinguished, but which potentially
contains the distinction in itself, to the actually distinct. It is
clear that the less an organ is developed, so much the more does it
approach the type, and that during its development it acquires more and
more peculiarities. The types discovered by comparative anatomy and
developmental history must therefore agree." And again, "What is true in
this idea is, that every embryo at first bears only the type of its
section, from which the type of the class, order, &c., is only
afterwards developed." Agassiz also observes that "the embryos of
different animals resemble each other the more the younger they are."

There are, no doubt, cases in which the earlier states are rapidly
passed through, or but obscurely indicated; yet we may almost state it
as a general proposition, that either before or after birth animals
undergo metamorphoses. The state of development of the young animal at
birth varies immensely. The kangaroo (_Macropus major_), which attains a
height of seven feet ten inches, does not when born exceed one inch and
two lines in length; the chick leaves the egg in a much more advanced
condition than the thrush; and so, among insects, the young cricket is
much more highly developed, when it leaves the egg, than the larva of
the fly or of the bee; and, as I have already mentioned, differences
occur even within the limit of one species, though not of course to
anything like the same extent.

In oviparous animals the condition of the young at birth depends much on
the size of the egg: where the egg is large, the abundant supply of
nourishment enables the embryo to attain a high stage of development;
where the egg is small, and the yolk consequently scanty, the embryo
requires an additional supply of food before it can do so. In the former
case the embryo is more likely to survive; but when the eggs are large,
they cannot be numerous, and a multiplicity of germs may be therefore in
some circumstances a great advantage. Even in the same species the
development of the egg presents certain differences.[43]

The metamorphoses of insects depend then primarily on the fact that the
young quit the egg at a more or less early stage of development; and
that consequently the external forces, acting upon them in this state,
are very different from those by which they are affected when they
arrive at maturity.

Hence it follows that, while in many instances mature forms, differing
greatly from one another, arise from very similar larvae, in other cases,
as we have seen, among some the parasitic Hymenoptera, insects agreeing
closely with one another, are produced from larvae which are very unlike.
The same phenomenon occurs in other groups. Thus, while in many cases
very dissimilar jelly-fishes arise from almost identical Hydroids, we
have also the reverse of the proposition in the fact that in some
species, Hydroids of an entirely distinct character produce very similar
Medusae.[44]

We may now pass to the second part of our subject: the apparent
suddenness and abruptness of the changes which insects undergo during
metamorphosis. But before doing so I must repeat that these changes are
not always, even apparently, sudden and great. The development of an
Orthopterous insect, say a grasshopper, from its leaving the egg to
maturity, is so gradual that the ordinary nomenclature of entomological
works (larva state and pupa state) does not apply to it; and even in the
case of Lepidoptera, the change from the caterpillar to the chrysalis
and from this to the butterfly is in reality less rapid than might at
first sight be supposed; the internal organs are metamorphosed very
gradually, and even the sudden and striking change in external form is
very deceptive, consisting merely of a throwing off of the outer
skin--the drawing aside, as it were of a curtain and the revelation of a
form which, far from being new, has been in preparation for days;
sometimes even for months.

Swammerdam, indeed, supposed (and his view was adopted by Kirby and
Spence) that the larva contained within itself "the germ of the future
butterfly, enclosed in what will be the case of the pupa, which is
itself included in three or more skins, one over the other, that will
successively cover the larva." This was a mistake; but it is true that,
if a larva be examined shortly before it is full grown, the future pupa
may be traced within it. In the same manner, if we examine a pupa which
is about to disclose the butterfly, we find the future insect, soft
indeed and imperfect, but still easily recognizable, lying more or less
loosely within the pupa-skin.

One important difference between an insect and a vertebrate animal is,
that whereas in the latter--as, for instance, in ourselves--the muscles
are attached to an internal bony skeleton, in insects no such skeleton
exists. They have no bones, and their muscles are attached to the skin;
whence the necessity for the hard and horny dermal investment of
insects, so different from the softness and suppleness of our own skin.
The chitine, or horny substance, of which the outside of an insect
consists, is formed by a layer of cells lying beneath it, and, once
secreted, cannot be altered. From this the result is, that without a
change of skin, a change of form is impossible. In some cases, as for
instance in _Chloeon_, each change of skin is accompanied by a change of
form, and thus the perfect insect is gradually evolved. In others, as in
caterpillars, several changes of skin take place without any material
alteration of form, and the change, instead of being spread over many,
is confined to the last two moults.

One explanation of this difference between the larvae which change their
form with every change of skin, and those which do not, is, I believe,
to be found in the structure of the mouth. That of the caterpillar is
provided with a pair of strong jaws, fitted to eat leaves; and the
digestive organs are adapted for this kind of food. On the contrary, the
mouth of the butterfly is suctorial; it has a long proboscis,
beautifully adapted to suck the nectar from flowers, but which would be
quite useless, and indeed only an embarrassment to the larva. The
digestive organs also of the butterfly are adapted for the assimilation,
not of leaves, but of honey. Now it is evident that if the mouth-parts
of the larva were slowly metamorphosed into those of the perfect insect,
through a number of small changes, the insect would in the meantime be
unable to feed, and liable to perish of starvation in the midst of
plenty. In the Orthoptera, and among those insects in which the changes
are gradual, the mouth of the so-called larva resembles that of the
perfect insect, and the principal difference consists in the presence of
wings.

Similar considerations throw much light on the nature of the chrysalis
or pupa state--that remarkable period of death-like quiescence which is
one of the most striking characteristics of insect metamorphosis. The
quiescence of the pupa is mainly owing to the rapidity of the changes
going on in it. In that of a butterfly, not only (as has been already
mentioned) are the mouth and the digestive organs undergoing change, but
the muscles are in a similar state of transition. The powerful ones
which move the wings are in process of formation; and even the nervous
system, by which the movements are set on foot and regulated, is in a
state of rapid change.[45]

It must not be forgotten that all insects are inactive for a longer or
shorter space of time after each moult. The slighter the change, as a
general rule, the shorter is the period of inaction. Thus, after the
ordinary moult of a caterpillar, the insect only requires a short rest
until the new skin is hardened. When, however, the change is great, the
period of inaction is correspondingly prolonged. Most pupae indeed have
some slight powers of motion; those which assume the chrysalis state in
wood or beneath the ground usually come to the surface when about to
assume the perfect state, and the aquatic pupae of certain Diptera swim
about with much activity. Among the Neuroptera, certain families have
pupae as quiescent as those of the Lepidoptera: others--as, for
instance, _Raphidia_--are quiescent at first, but at length acquire
sufficient strength to walk, though still enclosed within the pupa-skin:
a power dependent partly on the fact that this skin is very thin. Others
again--as, for instance, dragon-flies--are not quiescent on assuming the
so-called pupa state for any longer time than at their other changes of
skin. The inactivity of the pupa is therefore not a new condition
peculiar to this stage, but a prolongation of the inaction which has
accompanied every previous change of skin.

Nevertheless the metamorphoses of insects have always seemed to me one
of the greatest difficulties of the Darwinian theory. In most cases, the
development of the individual reproduces to a certain extent that of the
race; but the motionless, imbecile pupa cannot represent a mature form.
No one, so far as I know, has yet attempted to explain, in accordance
with Mr. Darwin's views, a life-history in which the mouth is first
mandibulate and then suctorial, as, for example, in a butterfly. A clue
to the difficulty may, I think, be found in the distinction between
developmental and adaptive changes; to which I have called attention in
a previous chapter. The larva of an insect is by no means a mere stage
in the development of the perfect animal. On the contrary, it is subject
to the influence of natural selection, and undergoes changes which have
reference entirely to its own requirements and condition. It is evident,
then, that while the embryonic development of an animal in the egg may
be an epitome of its specific history, this is by no means the case
with species in which the immature forms have a separate and
independent existence. If an animal which, when young, pursues one mode
of life, and lives on one kind of food, subsequently, either from its
own growth in size and strength, or from any change of season, alters
its habits or food, however slightly, it immediately becomes subject to
the action of new forces: natural selection affects it in two different,
and, it may be, very distinct manners, gradually tending to changes
which may become so great as to involve an intermediate period of change
and quiescence.

There are, however, peculiar difficulties in those cases in which, as
among the Lepidoptera, the same species is mandibulate as a larva, and
suctorial as an imago. From this point of view _Campodea_ and the
Collembola (_Podura_, &c.) are peculiarly interesting. There are in
insects three principal types of mouth:--

First, the mandibulate;

Secondly, the suctorial; and

Thirdly, that of _Campodea_ and the Collembola generally,

in which the mandibles and maxillae are retracted, but have some freedom
of motion, and can be used for biting and chewing soft substances. This
type is, in some respects, intermediate between the other two. Assuming
that certain representatives of such a type were placed under conditions
which made a suctorial mouth advantageous, those individuals in which
the mandibles and maxillae were best calculated to pierce or prick would
be favoured by natural selection, and their power of lateral motion
would tend to fall into abeyance; while, on the other hand, if
masticatory jaws were an advantage, the opposite process would take
place.

There is yet a third possibility--namely, that during the first portion
of life, the power of mastication should be an advantage, and during the
second that of suction, or _vice versa_. A certain kind of food might
abound at one season and fail at another; might be suitable for the
animal at one age and not at another. Now in such cases we should have
two forces acting successively on each individual, and tending to modify
the organization of the mouth in different directions. It cannot be
denied that the innumerable variations in the mouth-parts of insects
have special reference to their mode of life, and are of some advantage
to the species in which they occur. Hence, no believer in natural
selection can doubt the possibility of the three cases above suggested,
the last of which seems to throw some light on the possible origin of
species which are mandibulate in one period of life and not in another.
Granting then the transition from the one condition to the other, this
would no doubt take place contemporaneously with a change of skin. At
such times we know that, even when there is no change in form, the
softness of the organs temporarily precludes the insect from feeding for
a time, as, for instance, in the case of caterpillars. If, however, any
considerable change were involved, this period of fasting must be
prolonged, and would lead to the existence of a third condition, that of
the pupa, intermediate between the other two. Since the acquisition of
wings is a more conspicuous change than any relating to the mouth, we
are apt to associate with it the existence of a pupa-state: but the case
of the Orthoptera (grasshoppers, &c.) is sufficient proof that the
development of wings is perfectly compatible with permanent activity;
the necessity for prolonged rest is in reality much more intimately
connected with the change in the constitution of the mouth, although in
many cases, no doubt, this is accompanied by changes in the legs, and in
the internal organization. An originally mandibulate mouth, however,
like that of a beetle, could not, I think, have been directly modified
into a suctorial organ like that of a butterfly or a gnat, because the
intermediate stages would necessarily be injurious. Neither, on the
other hand, for the same reasons, could the mouth of the Hemiptera be
modified into a mandibulate type like that of the Coleoptera. But in
_Campodea_ and the _Collembola_ we have a type of animal closely
resembling certain larvae which occur both in the mandibulate and
suctorial series of insects, possessing a mouth neither distinctly
mandibulate nor distinctly suctorial, but constituted on a peculiar
type, capable of modification in either direction by gradual change,
without loss of utility.

In discussing this subject, it is necessary also to take into
consideration the nature and origin of wings. Whence are they derived?
why are there normally two pairs? and why are they attached to the
meso-and meta-thorax? These questions are as difficult as they are
interesting. It has been suggested, and I think with justice, that the
wings of insects originally served for aquatic and respiratory
purposes.

In the larva of _Chloeon_ (Pl. IV., Fig. 1), for instance, which in
other respects so singularly resembles _Campodea_ (Pl. III., Fig. 5),
several of the segments are provided with foliaceous expansions which
serve as respiratory organs. These so-called branchiae are in constant
agitation, and the muscles which move them in several points resemble
those of true wings. It is true that in _Chloeon_ the vibration of the
branchiae is scarcely, if at all, utilized for the purpose of locomotion;
the branchiae are, in fact, placed too far back to act efficiently. The
situation of these branchiae differs in different groups; indeed, it
seems probable that originally there were a pair on each segment. In
such a case, those branchiae situated near the centre of the body,
neither too much in front nor too far back, would serve the most
efficiently as propellers: the same causes which determined the position
of the legs would also affect the wings. Thus a division of labour would
be effected; the branchiae on the thorax would be devoted to locomotion;
those on the abdomen to respiration. This would tend to increase the
development of the thoracic segments, already somewhat enlarged, in
order to receive the muscles of the legs.

That wings may be of use to insects under water is proved by the very
interesting case of _Polynema natans_,[46] which uses its wings for
swimming. This, however, is a rare case, and it is possible that the
principal use of the wings was, primordially, to enable the mature forms
to pass from pond to pond, thus securing fresh habitats and avoiding
in-and-in breeding. If this were so, the development of wings would
gradually have been relegated to a late period of life; and by the
tendency to the inheritance of characters at corresponding ages, which
Mr. Darwin has pointed out,[47] the development of wings would have thus
become associated with the maturity of the insect. Thus the late
acquisition of wings in the Insecta generally seems to be itself an
indication of their descent from a stock which was at one period, if not
originally, aquatic, and which probably resembled the present larvae of
_Chloeon_ in form, but had thoracic as well as abdominal branchiae.

Finally, from the subject of metamorphosis we pass naturally to that
most remarkable phenomenon which is known as the "Alternation of
Generations:" for the first systematic view of which we are indebted to
my eminent friend Prof Steenstrup.[48]

I have always felt it very difficult to understand why any species
should have been created in this double character; nor, so far as I am
aware, has any explanation of the fact yet been attempted. Nevertheless
insects offer, in their metamorphoses, a phenomenon not altogether
dissimilar, and give a clue to the manner in which alternation of
generations may have originated.

The caterpillar owes its difference from the butterfly to the
undeveloped state in which it leaves the egg; but its actual form is
mainly due to the influence of the conditions under which it lives. If
the caterpillar, instead of changing into one butterfly, produced
several, we should have an instance of alternation of generations. Until
lately, however, we knew of no such case among insects; each larva
produced one imago, and that not by generation, but by development. It
has long been known, indeed, that there are species in which certain
individuals remain always apterous, while others acquire wings. Many
entomologists, however, regard these abnormal individuals as perfect,
though wingless insects; and therefore I shall found no argument upon
these cases, although they appear to me deserving of more attention than
they have yet received.

Recently, however, Prof. Wagner[49] has discovered that, among certain
small gnats, the larvae do not directly produce in all cases perfect
insects, but give birth to other larvae, which undergo metamorphoses of
the usual character, and eventually become gnats. His observations have
been confirmed, as regards this main fact, by other naturalists; and
Grimm has met with a species of _Chironomus_ in which the pupae lay
eggs.[50]

Here, then, we have a distinct case of alternation of generations, as
characterized by Steenstrup. Probably other cases will be discovered in
which insects undeniably in the larval state will be found fertile. Nay,
it seems to me possible, if not probable, that some larvae which do not
now breed may, in the course of ages, acquire the power of doing so. If
this idea is correct, it shows how the remarkable phenomenon, known as
alternation of generations, may have originated.

Summing up, then, the preceding argument, we find among insects various
modes of development; from simple growth on the one hand, to well-marked
instances of the so-called alternation of generation on the other. In
the wingless species of Orthoptera there is little external difference,
excepting in size, between the young larva and the perfect insect. The
growth is gradual, and there is nothing which would, in ordinary
language, be called a metamorphosis. In the majority of Orthoptera,
though the presence of wings produces a marked difference between the
larva and the imago, the habits are nearly the same throughout life, and
consequently the action of external circumstances affects the larva in
the same manner as it does the perfect insect.

This is not the case with the Neuroptera. The larvae do not live under
the same conditions as the perfect insects: external forces accordingly
affect them in a different manner; and we have seen that they pass
through some changes which bear no reference to the form of the perfect
insect: these changes, however, are for the most part very gradual. The
caterpillars of Lepidoptera have even more extensive modifications to
undergo; the mouth of the larva, for instance, being remarkably unlike
that of the perfect insect. A change in this organ, however, could
hardly take place while the insect was growing fast, and consequently
feeding voraciously; nor, even if the change could be thus effected,
would the mouth, in its intermediate stages, be in any way fitted for
biting and chewing leaves. The same reasoning applies also to the
digestive organs. Hence the caterpillar undergoes little, if any,
change, except in size, and the metamorphosis is concentrated, so to
say, into the last two moults. The changes then become so rapid and
extensive, that the intermediate period is necessarily one of
quiescence. In some exceptional cases, as in _Sitaris_ (_ante_, p. 30) we
even find that, the conditions of life not being uniform throughout the
larval period, the larva itself undergoes metamorphoses.

Owing to the fact that the organs connected with the reproduction of the
species come to maturity at a late period, larvae are generally incapable
of breeding. There are, however, some flies which have viviparous larvae,
and thus offer a typical case of alternation of generations.

Thus, then, we find among insects every gradation, from simple growth to
alternation of generations; and see how, from the single fact of the
very early period of development at which certain animals quit the egg,
we can throw some light on their metamorphoses, and for the still more
remarkable phenomenon that, among many of the lower animals, the species
is represented by two very different forms. We may even conclude, from
the same considerations, that this phenomenon may in the course of ages
become still more common than it is at present. As long, however, as the
external organs arrive at their mature form before the internal
generative organs are fully developed, we have metamorphosis; but if the
reverse is the case, then alternation of generations often results.

The same considerations throw much light on the remarkable circumstance,
that in alternation of generations the reproduction is, as a general
rule, agamic in one form. This results from the fact that reproduction
by distinct sexes requires the perfection both of the external and
internal organs; and if the phenomenon arise, as has just been
suggested, from the fact that the internal organs arrive at maturity
before the external ones, reproduction will result in those species only
which have the power of agamic multiplication.

Moreover, it is evident that we have in the animal kingdom two kinds of
dimorphism.

This term has usually been applied to those cases in which animals or
plants present themselves at maturity under two forms. Ants and Bees
afford us familiar instances among animals; and among plants the
interesting case of the genus _Primula_ has recently been described by
Mr. Darwin. Even more recently he has made known to us the still more
remarkable phenomenon afforded by the genus _Lythrum_, in which there
are three distinct forms, and which therefore offers an instance of
polymorphism.[51]

The other kind of dimorphism or polymorphism differs from the first in
being the result of the differentiating action of external
circumstances, not on the mature, but on the young individual. Such
different forms, therefore, stand towards one another in the relation of
succession. In the first kind the chain of being divides at the
extremity; in the other it is composed of dissimilar links. Many
instances of this second form of dimorphism have been described under
the name of alternation of generations.

The term, however, has met with much opposition, and is clearly
inapplicable to the differences exhibited by insects in various periods
of their life. Strictly speaking, the phenomena are frequently not
alternate, and in the opinion of some eminent naturalists they are not,
strictly speaking, cases of generation at all.[52]

In order, then, to have some name for these remarkable phenomena, and to
distinguish them from those cases in which the _mature_ animal or plant
is represented by two or more different forms, I think it would be
convenient to retain exclusively for these latter the terms dimorphism
and polymorphism; and those cases in which animals or plants pass
through a succession of different forms might be distinguished by the
name of dieidism or polyeidism.

The conclusions, then, which I think we may draw from the preceding
considerations, are:--

1. That the occurrence of metamorphoses arises from the immaturity of
the condition in which some animals quit the egg.

2. That the form of the insect larva depends in great measure on the
conditions in which it lives. The external forces acting upon it are
different from those which affect the mature form; and thus changes are
produced in the young, having reference to its immediate wants, rather
than to its final form.

3. That metamorphoses may therefore be divided into two kinds,
developmental and adaptional or adaptive.

4. That the apparent abruptness of the changes which insects undergo,
arises in great measure from the hardness of their skin, which admits of
no gradual alteration of form, and which is itself necessary in order to
afford sufficient support to the muscles.

5. The immobility of the pupa or chrysalis depends on the rapidity of
the changes going on in it.

6. Although the majority of insects go through three well-marked stages
after leaving the egg, still a large number arrive at maturity through a
greater or smaller number of slight changes.

7. When the external organs arrive at this final form before the organs
of reproduction are matured, these changes are known as metamorphoses;
when, on the contrary, the organs of reproduction are functionally
perfect before the external organs, or when the creature has the power
of budding, then the phenomenon is known as alternation of generations.




CHAPTER V.

_ON THE ORIGIN OF INSECTS._


"Personne," says Carl Vogt, "en Europe au moins, n'ose plus soutenir la
Creation independante et de toutes pieces des especes," and though this
statement is perhaps not strictly correct, still it is no doubt true,
that the Doctrine of Evolution, in some form or other, is accepted by
most, if not by all, the greatest naturalists of Europe. Yet it is
surprising how much, in spite of all that has been written, Mr. Darwin's
views are still misunderstood. Thus Browning, in one of his recent
poems, says:--

  "That mass man sprang from was a jelly lump
   Once on a time; he kept an after course
   Through fish and insect, reptile, bird, and beast,
   Till he attained to be an ape at last,
   Or last but one."[53]

This theory, though it would be regarded by many as a fair statement of
his views, is one which Mr. Darwin would entirely repudiate. Whether
fish and insect, reptile, bird and beast, are derived from one original
stock or not, they are certainly not links in one sequence. I do not,
however, propose to discuss the question of Natural Selection, but may
observe that it is one thing to acknowledge that in Natural Selection,
or the survival of the fittest, Mr. Darwin has called attention to a
_vera causa_, has pointed out the true explanation of certain phenomena;
but it is quite another thing to maintain that all animals are descended
from some primordial source.

For my own part, I am satisfied that Natural Selection is a true cause,
and, whatever may be the final result of our present inquiries--whether
animated nature be derived from one ancestral source, or from many--the
publication of the Origin of Species will none the less have constituted
an epoch in the History of Biology. But, how far the present condition
of living beings is due to that cause; how far, on the other hand, the
action of Natural Selection has been modified and checked by other
natural laws--by the unalterability of types, by atavism, &c.; how many
types of life originally came into being; and whether they arose
simultaneously or successively,--these and many other similar questions
remain unsolved, even admitting the theory of Natural Selection. All
this has indeed been clearly pointed out by Mr. Darwin himself, and
would not need repetition but for the careless criticism by which in too
many cases the true question has been obscured. Without, however,
discussing the argument for and against Mr. Darwin's conclusions, so
often do we meet with travesties of it like that which I have just
quoted, that it is well worth while to consider the stages through which
some group, say for instance that of insects, have probably come to be
what they are, assuming them to have developed under natural laws from
simpler organisms. The question is one of great difficulty. It is hardly
necessary to say that insects cannot have passed through all the lower
forms of animal life, and naturalists do not at present agree as to the
actual line of their development.

In the case of insects, the gradual course of evolution through which
the present condition of the group has probably been reached, has been
discussed by Mr. Darwin, by Fritz Mueller, Haeckel, Brauer, myself and
others.

In other instances Palaeontology throws much light on this question.
Leidy has shown that the milk-teeth of the genus _Equus_ resemble the
permanent teeth of the ancient _Anchitherium_, while the milk-teeth of
_Anchitherium_ again approximate to the dental system of the still
earlier _Merychippus_. Ruetimeyer, while calling attention to this
interesting observation, adds that the milk-teeth of _Equus caballus_ in
the same way, and still more those of _E. fossilis_, resemble the
permanent teeth of _Hipparion_.

"If we were not acquainted with the horse," says Flower,[54] "we could
scarcely conceive of an animal whose only support was the tip of a
single toe on each extremity, to say nothing of the singular
conformation of its teeth and other organs. So striking have these
characters appeared to many zoologists, that the animals possessing them
have been reckoned as an order apart, called Solidungula; but
palaeontology has revealed that in the structure of its skull, its teeth,
its limbs, the horse is nothing more than a modified _Palaeotherium_; and
though still with gaps in certain places, many of the intermediate
stages of these modifications are already known to us, being the
_Palaeotherium_, _Anchitherium_, _Merychippus_, and _Hipparion_."

"All Echinoids," says A. Agassiz,[55] "pass, in their early stages,
through a condition which recalls to us the first Echinoids which made
their appearance in geological ages." On embryological grounds, he
observes, we should "place true Echini lowest, then the Clypeastroids,
next the Echinolamps, and finally the Spatangoids." Now among the
Echinoids of the Trias there are no Clypeastroids, Echinolamps, or
Spatangoids. The Clypeastroids make their appearance in the Lias, the
Echinolamps in the Jurassic, while the Spatangoids commence in the
Cretaceous period.

Again[56] "in the Radiates, the Acalephs in their first stages of
growth, that is, in their Hydroid condition, remind us of the adult
forms among Polyps, showing the structural rank of the Acalephs to be
the highest, since they pass beyond a stage which is permanent with the
Polyps; while the Adult forms of the Acalephs have in their turn a
certain resemblance to the embryonic phases of the class next above
them, the Echinoderms; within the limits of the classes, the same
correspondence exists as between the different orders; the embryonic
forms of the highest Polyps recall the adult forms of the lower ones,
and the same is true of the Acalephs as far as these phenomena have been
followed and compared among them." Indeed, the accomplished authors from
whom I have taken the above quotation, do not hesitate to say[57] that
"whenever such comparisons have been successfully carried out, the
result is always the same; the present representatives of the fossil
types recall in their embryonic condition the ancient forms, and often
explain their true position in the animal kingdom."

Fossil insects are unfortunately rare, there being but few strata in
which the remains of this group are well preserved. Moreover,
well-characterized Orthoptera and Neuroptera occur as early as the
Devonian strata; Coleoptera and Hemiptera in the Coal-measures;
Hymenoptera and Diptera in the Jurassic; Lepidoptera, on the contrary,
not until the Tertiary. But although it appears from these facts that,
as far as our present information goes, the Orthoptera and Neuroptera
are the most ancient orders, it is not, I think, conceivable that the
latter should have been derived from any known species of the former; on
the other hand, the earliest known Neuroptera and Orthoptera, though in
some respects less specialized than existing forms, are as truly, and as
well characterized, Insects, as any now existing; nor are we acquainted
with any earlier forms, which in any way tend to bridge over the gap
between them and lower groups, though, as we shall see, there are types
yet existing which throw much light on the subject.

In the consideration then of this question, we must rely principally on
Embryology and Development. I have already referred to the cases in
which species, very unlike in their mature condition, are very
similar one to another when young. Haeckel, in his "Naturliche
Schoepfungsgeschichte," gives a diagram which illustrates this very well
as regards Crustacea. Pls. 1-4 show the same to be the case with
Insects.

The Stag-beetle, the Dragon-fly, the Moth, the Bee, the Ant, the Gnat,
the Grasshopper,--these and other less familiar types seem at first to
have little in common. They differ in size, in form, in colour, in
habits, and modes of life. Yet the researches of entomologists,
following the clue supplied by the illustrious Savigny, have proved, not
only that while differing greatly in details, they are constructed on
one common plan; but also that other groups, as for instance, Crustacea
(Lobsters, Crabs, &c.) and Arachnida (Spiders and Mites), can be shown
to be fundamentally similar. In Pl. 4 I have figured the larvae of an
_Ephemera_ (Fig. 1), of a _Meloe_ (Fig. 2), of a Dragon-fly (Fig. 3), of a
Sitaris (Fig. 4), of a _Campodea_ (Fig. 5), of a _Dyticus_ (Fig. 6), of a
Termite (Fig. 7), of a _Stylops_ (Fig. 8), and of a _Thrips_ (Fig. 9). All
these larvae possess many characters in common. The mature forms are
represented in the corresponding figures of Plate 3, and it will at once
be seen how considerably they differ from one another. The same fact is
also illustrated in Figs. 48-55, where Figs. 48-51 represent the larval
states of the mature forms represented in Figs. 52-55. Fig. 48 is the
larva of a moth, _Agrotis suffusa_ (Fig. 52); Fig. 49 of a beetle,
_Haltica_ (Fig. 53); Fig. 50 of a Saw-fly, _Cimbex_ (Fig. 54); and Fig.
51 of a Centipede, _Julus_ (Fig. 55).

[Illustration: FIG. 48, Larva of Moth (_Agrotis suffusa_), after
Packard. 49, Larva of Beetle (_Haltica_), after Westwood. 50, Larva of
Saw-fly (_Cimbex_), Brischke and Zaddach. Beob. ub d. arten. der Blatt
und Holzwespen, Fig. 8. 51, Larva of _Julus_. Newport, Philos.
Transactions, 1841.]

Thus, then, although it can be demonstrated that perfect insects,
however much they differ in appearance, are yet reducible to one type,
the fact becomes much more evident if we compare the larvae. M.
Brauer[58] and I[59] have pointed out that two types of larvae, which I
have proposed to call _Campodea_-form and _Lindia_-form, and which Packard
has named Leptiform and Eruciform, run through the principal groups of
insects. This is obviously a fact of great importance: as all individual
_Meloes_ are derived from a form resembling Pl. 2, Fig. 2, it is surely no
rash hypothesis to suggest that the genus itself may have been so.

[Illustration: FIG. 52, _Agrotis suffusa_ (after Packard). 53, _Haltica_
(after Westwood).]

[Illustration: FIG. 54, _Cimbex_, Brischae and Zaddach. l.c. T. 2, Fig.
9.]

[Illustration: FIG. 55. _Julus_ (after Gervais).]

Firstly, however, let me say a word as to the general Insect type. It
may be described shortly as consisting of animals possessing a head,
with mouth parts, eyes and antennae; a many segmented body, with three
pairs of legs on the segments immediately following the head; with, when
mature, either one or two pairs of wings, generally with caudal
appendages I will not now enter into a description of their internal
anatomy. It will be seen that, except as regards the wings, Pl. 4, Fig.
4, representing the larva of a small beetle named _Sitaris_, answers very
well to this description. Many other Beetles are developed from larvae
closely resembling those of _Meloe_ (Pl. 4, Fig. 2), and Sitaris (Pl. 4,
Fig. 4); in fact--except those species the larvae of which, as, for
instance of the Weevils (Pl. 2, Fig. 6), are internal feeders, and do
not require legs--we may say that the Coleoptera generally are derived
from larvae of this type.

I will now pass to a second order, the Neuroptera. Pl. 4, Fig. 1,
represents the larva of _Chloeon_, a species the metamorphoses of which I
described some years ago in the Linnean Transactions,[60] and it is
obvious that in essential points it closely resembles the form to which
I have just alluded.

The Orthoptera, again, the order to which Grasshoppers, Crickets,
Locusts, &c. belong, commence life in a similar condition; and the same
may also be said of the Trichoptera.

The larvae of Bees when they quit the egg are entirely legless, but in an
earlier stage they possess well-marked rudiments of thoracic legs,
showing, as it seems to me, that their apodal condition is an adaptation
to their circumstances. Other Hymenopterous larvae, those for example of
_Sirex_ (Fig. 9), and of the Saw-flies (Fig. 50) have well-developed
thoracic legs.

From the difference in external form, and especially from the large
comparative size of the abdomen, these larvae, as well as those of
Lepidoptera (Fig. 48), have generally been classed with the maggots of
Flies, Weevils, &c., rather than with the more active form of larva just
adverted to. This seems to me, as I have already pointed out,[61] to be
a mistake. The caterpillar type differs, no doubt, in its general
appearance, owing to its greater clumsiness, but still essentially
agrees with that already described.

No Dipterous larva, so far as I know, belongs truly to this type; in
fact, the early stages of the pupa in the Diptera seem in some respects
to correspond to the larvae of other Insect orders. The Development of
the Diptera is, however, as Weissman[62] has shown, very abnormal in
other respects.

Thus, then, we find in many of the principal groups of insects that,
greatly as they differ from one another in their mature condition, when
they leave the egg they more nearly resemble the typical insect type;
consisting of a head; a three-segmented thorax, with three pairs of
legs; and a many-jointed abdomen, often with anal appendages. Now, is
there any mature animal which answers to this description? We need not
have been surprised if this type, through which it would appear that
insects must have passed so many ages since (for winged Neuroptera have
been found in the carboniferous strata) had long ago become extinct. Yet
it is not so. The interesting genus _Campodea_ (Pl. 3, Fig. 5) still
lives; it inhabits damp earth, and closely resembles the larva of
_Chloeon_ (Pl. 2, Fig. 1), constituting, indeed, a type which, as shown in
Pl. 4, occurs in many orders of insects. It is true that the
mouth-parts of _Campodea_ do not resemble either the strongly mandibulate
form which prevails among the larvae of Coleoptera, Orthoptera,
Neuroptera, Hymenoptera, Lepidoptera; or the suctorial type of the
Homoptera and Heteroptera. It is, however, not the less interesting or
significant on that account, since, as I have elsewhere[63] pointed out,
its mouth-parts are intermediate between the mandibulate and haustellate
types; a fact which seems to me most suggestive.

It appears, then, that there are good grounds for considering that the
various types of insects are descended from ancestors more or less
resembling the genus _Campodea_, with a body divided into head, thorax,
and abdomen: the head provided with mouth-parts, eyes, and one pair of
antennae; the thorax with three pairs of legs; and the abdomen, in all
probability, with caudal appendages.

If these views are correct, the genus _Campodea_ must be regarded as a
form of remarkable interest since it is the living representative of a
primaeval type, from which not only the Collembola and Thysanura, but the
other great orders of insects have derived their origin.

From what lower group the _Campodea_ type was itself derived is a question
of great difficulty. Fritz Mueller indeed says,[64] "if all the classes
of Arthropoda (Crustacea, Insecta, Myriopoda, and Arachnida) are indeed
all branches of a common stem (and of this there can scarcely be a
doubt), it is evident that the water-inhabiting and water-breathing
Crustacea must be regarded as the original stem from which the other
terrestrial classes, with their tracheal respiration, have branched
off." Haeckel, moreover, is of the opinion that the Tracheata are
developed from the Crustacea, and probably from the Zoepoda. For my own
part, though I feel very great diffidence in expressing an opinion at
variance with that of such high authorities, I am rather disposed to
suggest that the _Campodea_ type may possibly have been derived from a
less highly developed one, resembling the modern Tardigrade,[65] a (Fig.
56) smaller and much less highly organized being than _Campodea_. It
possesses two eyes, three anterior pairs of legs, and one at the
posterior end of the body, giving it a curious resemblance to some
Lepidopterous larvae.

[Illustration: FIG. 56, Tardigrade (after Dujardin).]

These legs, however, as will be seen, are reduced to mere projections.
But for them, the Tardigrada would closely resemble the vermiform larva
so common among insects. Among Trichoptera the larva early acquires
three pairs of legs, but as Zaddach has shown,[66] there is a stage,
though it is quickly passed through, in which the divisions of the body
are indicated, but no trace of legs is yet present. Indeed, there appear
to be reasons for considering that while among Crustacea the appendages
appear before the segments, in Insects the segments precede the
appendages, although this stage of development is very transitory, and
apparently, in some cases, altogether suppressed. I say "apparently,"
because, as I have already mentioned, I am not yet satisfied that it
will not eventually be found to be so in all cases. Zaddach, in his
careful observations of the embryology of _Phryganea_, only once found a
specimen in this stage, which also, according to the researches of
Huxley,[67] seems to be little more than indicated in _Aphis_. It is
therefore possible that in other cases, when no such stage has been
observed, it not really may be absent, but, from its transitoriness, may
have hitherto escaped attention.

Fritz Mueller has expressed the opinion[68] that this vermiform type is
of comparatively recent origin. He says: "The ancient insects approached
more nearly to the existing Orthoptera, and perhaps to the wingless
Blattidae, than to any other order, and the complete metamorphosis of the
Beetles, Lepidoptera, &c., is of later origin." "There were," he adds,
"perfect insects before larvae and pupae." This opinion has been adopted
by Mr. Packard[69] in his "Embryological Studies on Hexapodous Insects."

M. Brauer[70] also considers that the vermiform larva is a more recent
type than the Hexapod form, and is to be regarded not as a developmental
form, but as an adaptational modification of the earlier active hexapod
type. In proof of this he quotes the case of _Sitaris_.

Considering, however, the peculiar habits of this genus, to which I have
already referred, and also that the vermiform type is altogether lower
in organization and less differentiated than the _Campodea_ form, I cannot
but regard this case as exceptional; one in which the development has
been, as it were, to use an expression of Fritz Mueller's, "falsified" by
the struggle for existence, and which therefore does not truly indicate
the successive stages of evolution. On the whole, the facts seem to me
to point to the conclusion that, though the grub-like larvae of Coleoptera
and some other insects, owe their present form mainly to the influence
of external circumstances, and partially also to atavism, still the
_Campodea_ type is itself derived from earlier vermiform ancestors.
Nicolas Wagner has shown in the case of a small gnat, allied to
_Cecidomyia_, that even now, in some instances, the vermiform larvae
possess the power of reproduction. Such a larva (as, for instance, Fig.
57) very closely resembles some of the Rotatoria, such for instance as
_Albertia_ or _Notommata_, which however possess vibratile cilia. There is,
indeed, one genus--_Lindia_ (Fig. 58)--in which these ciliae are altogether
absent, and which, though resembling _Macrobiotus_ in many respects,
differs from that genus in being entirely destitute of legs. I have
never met with it myself, but it is described by Dujardin, who found it
in a ditch near Paris, as being oblong, vermiform, divided into rings,
and terminating posteriorly in two short conical appendages. The jaws
are not unlike those of the larvae of Flies, and indeed many naturalists
meeting with such a creature would, I am sure, regard it as a small
Dipterous larva; yet Dujardin figures a specimen containing an egg, and
seems to have no doubt that it is a mature form.[71]

For the next descending stage we must, I think, look among the
Infusoria, through such genera as _Chaetonotus_ or _Ichthydium_. Other
forms of the Rotatoria, such for instance as _Rattulus_, and still more
the very remarkable species discovered in 1871 by Mr. Hudson,[72] and
described under the name of _Pedalion mira_, seem to lead to the
Crustacea through the Nauplius form. Dr. Cobbold tells me that he
regards the _Gordii_ as the lowest of the Scolecida; Mr. E. Ray
Lankester considers some of the Turbellaria, such genera as
_Mesostomum_, _Vortex_, &c., to be the lowest of existing worms;
excluding the parasitic groups. Haeckel[73] also regards the Turbellaria
as forming the nearest approach to the Infusoria. The true worms seem,
however, to constitute a separate branch of the animal kingdom.

We may take, as an illustration of the lower worms, the genus
Prorhynchus (Fig. 59), which consists of a hollow cylindrical body,
containing a straight simple tube, the digestive organ.

But however simple such a creature as this may be, there are others
which are far less complex, far less differentiated; which therefore, on
Mr. Darwin's principles, may be considered still more closely to
represent the primaeval ancestor from which these more highly-developed
types have been derived, and which, in spite of their great
antiquity--in spite of, or perhaps in consequence of, their simplicity,
still maintain themselves almost unaltered.

Thus the form which Haeckel has described[74] under the name
_Protamoeba primitiva_, Pl. 5, Fig. 1-5, consists of a homogeneous and
structureless substance, which continually alters its form; putting out
and drawing in again more or less elongated processes, and creeping
about like a true _Amoeba_, from which, however, _Protamoeba_ differs,
in the absence of a nucleus. It seems difficult to imagine anything
simpler; indeed, as described, it appears to be an illustration of
properties without structure. It takes into itself any suitable particle
with which it comes in contact, absorbs that which is nutritious, and
rejects the rest. From time to time a constriction appears at the centre
(Pl. 5, Fig. 2), its form approximates more and more to that of an
hour-glass (Pl. 5, Fig. 3), and at length the two halves separate, and
each commences an independent existence (Pl. 5, Fig. 5).

[Illustration: FIG. 59, _Prorhynchus stagnaus_.[75]]

[Illustration: PLATE V.

FIGS. 1-5, _Protamoeba_; 6-9, _Protamyxa Aurantiaca_,
Haeckel, Beit. zur Monog. der Moneren, pl. 1; 10-18, _Magosphaera planula_,
Haeckel, loc. cit. pl. 5.]

In the true _Amoebas_, on the contrary, we find a differentiation
between the exterior and the interior: the body being more or less
distinctly divisible into an outer layer and an inner parenchyme. In the
_Amoebas_, as in _Protamoeba_, multiplication takes place by
self-division, and nothing corresponding to sexual reproduction has yet
been discovered.

Somewhat more advanced, but still of great simplicity, is the _Protomyxa
aurantiaca_ (Pl. 5, Fig. 8), discovered by Haeckel[76] on dead shells of
_Spirula_, where it appears as a minute orange speck, which shows well
against the clear white of the _Spirula_. Examined with a microscope, the
speck is seen to be a spherical mass of orange-, homogeneous,
albuminous matter, surrounded by a delicate, structureless membrane. It
is obvious from this description that these bodies closely resemble
eggs, for which indeed Haeckel at first mistook them. Gradually,
however, the yellow sphere broke itself up into smaller spherules (Pl.
5, Fig. 9), after which the containing membrane burst, and the separate
spherules, losing their globular form, crept out as small _Amoebae_ (Pl.
5, Fig. 6), or amoeboid bodies. These little bodies moved about,
assimilated the minute particles of organic matter, with which they came
in contact, and gradually increased in size (Pl. 5, Fig. 7) with more or
less rapidity according to the amount of nourishment they were able to
obtain. They threw out arms in various directions, and if divided each
section maintained its individual existence. After a while their
movements ceased, they contracted into a ball, and again secreted round
themselves a clear structureless envelope.

This completes their life history as observed by Haeckel, who found it
easy to retain them in his glasses in perfect health, and who watched
them closely.

As another illustration I may take the _Magosphaera planula_, discovered
by Haeckel on the coast of Norway.

In one stage of its existence (Pl. 5, Fig. 10) it is a minute mass of
gelatinous matter, which continually alters its form, moves about,
feeds, and in fact behaves altogether like the _Amoeba_ just described.
It does not, however, remain always in this condition. After a while it
contracts into a spherical form (Pl. 5, Fig. ii), and secretes round
itself a structureless envelope, which, with the nucleus, gives it a
very close resemblance to a minute egg.

Gradually the nucleus divides, and the protoplasm also separates into
two spherules (Pl. 5, Fig. 12); these two subdivide into four (Pl. 5,
Fig. 13), and so on (Pl. 5, Fig 14), until at length thirty-two are
present, compressed into a more or less polygonal form (Pl. 5, Fig. 15).
Here this process ends. The separate spherules now begin to lose their
smooth outline, to throw out processes, and to show amoeboid movements
like those of the creatures just described. The processes or pseudopods
grow gradually longer, thinner, and more pointed. Their movements become
more active, until at length they take the form of ciliae. The spherical
_Magosphaera_, the upper surface of which has thus become covered with
ciliae, now begins to rotate within the cyst or envelope, which at length
gives way and sets free the contained sphere, which then swims about
freely in the water (Pl. 5, Fig. 16), thus closely resembling _Synura_, or
one of the Volvocineae. After swimming about in this condition for a
certain time, the sphere breaks up into the separate cells of which it
is composed (Pl. 5, Fig. 17). As long as the individual cells remained
together, they had undergone no changes of form, but after separating
they show considerable contractility, and gradually alter their form,
until they become undistinguishable from true _Amoebae_ (Pl. 5, Fig. 18).
Finally, according to Haeckel, these amoeboid bodies, after living for
a certain time in this condition, return to a state of rest, again
contract into a spherical form, and secrete round themselves a
structureless envelope. The life history of some other low organisms, as
for instance _Gregarina_, is of a similar character.

It may be said, and said truly, that the difference between such beings
as these and the _Campodea_, or Tardigrade, is immense. But if it be
considered incredible that even during the long lapse of geological time
such great changes should have taken place as are implied in the belief
that there is genetic connection between them and these lower groups,
let us consider what happens under our eyes in the development of each
one of these little creatures in the proverbially short space of their
individual life.

I will take for instance the first stages, and for the sake of brevity
only the first stages, of the life-history of a Tardigrade.[77] As shown
in Fig. 60, the egg is at first a round body or cell, with a clear
central nucleus--the germinal vesicle; it increases in size, and after
a while the yolk and the germinal vesicle divide into two (Fig. 61),
then into four (Fig. 62), and so on, just as we have seen to be the case
in _Magosphaera_. From the minute cells (Fig. 63) arising through this
process of yolk-segmentation, the body of the Tardigrade is then built
up.[78]

[Illustration: FIG. 60, Egg of Tardigrade, Kaufmann, Zeit f. Wiss.
Zool. 1851, Pl. 1. 61, Egg of Tardigrade after the yolk has subdivided.
62, Egg of Tardigrade in the next stage. 63, Egg of Tardigrade more
advanced.]

Though I will not now attempt to point out the full bearing of these
facts on the study of embryology generally, yet I cannot resist calling
attention to the similarity of the development of _Magosphaera_ with the
first stages of development of other animals, because it appears to me
to possess a significance, the importance of which it would be difficult
to overestimate.

Among the Zoophytes Prof. Allman thus describes[79] the process in
_Laomedea_, as representing the Hydroids (Pl. 6, Fig. 1, represents the
young egg):--"The first step observable in the segmentation-process is
the cleavage of the yolk into two segments (Pl. 6, Fig. 2), immediately
followed by the cleavage of these into other two, so that the vitellus
is now composed of four cleavage spheres (Pl. 6, Fig. 3)." These spheres
again divide (Pl. 6, Fig. 4) and subdivide, thus at length forming
minute cells, of which the body of the embryo is built up.

In Pl. 6, Figs. 5-9 represent the corresponding stages in the
development of a small parasitic worm--the _Filaria mustelarum_--as
given by Van Beneden.[80] The first process is that within the egg,
which represents, so to say, the encysted condition of _Magosphaera_, the
yolk divides itself into two balls (Pl. 6, Fig. 6), then into four,
eight, and so on, the cells thus constituted finally forming the young
worm. I have myself observed the same stages in the eggs of the very
remarkable and abnormal _Sphaerularia bombi_.[81]

Among the Echinoderms M. Derbes thus describes the first stages (Pl. 6,
Figs. 10-13) in the development of the egg of an _Echinus_ (_Echinus
esculentus_):--"Le jaune commence a se segmenter, d'abord en deux, puis
en quatre et ainsi de suite, chacune des nouvelles cellules se
partageant a son tour en deux."[82] Sars has observed the same thing in
the starfish.[83]

[Illustration: PLATE. 6.]

In the Rotatoria, as shown by Huxley in _Lacinularia_,[84] and by
Williamson in _Melicerta_,[85] the yolk is at first a single globular
mass, the first changes which take place in it being as follows:--"The
central nucleus becomes drawn out and subdivides into two, this division
being followed by a corresponding segmentation of the yolk. The same
process is repeated again and again, until at length the entire yolk is
converted into a mass of minute cells." Among the Crustacea the total
segmentation of the yolk occurs among the Copepoda, Rhizocephala, and
Cirripedia. Sars has described the same process in one of the
nudibranchiate mollusca[86] (_Tritonia_), Mueller in Entochocha,[87]
Haeckel in Ascidia,[88] Lacaze Duthiers in _Dentalium_.[89] Figures 18 to
21, Pl. 6, are taken from Koren and Danielssen's[90] memoir on the
development of _Purpura lapillus_.

Figs. 22-24 show the same stages in a fish (_Amphioxus_) as given by
Haeckel, and it is unnecessary to point out the great similarity.

Lastly, figures 25 to 29, Pl. 6, are given by Dr. Allen Thomson,[91] as
illustrating the first stages in the development of the vertebrata.

I might have given many other examples, but the above are probably
sufficient, and will show that the processes which constitute the
life-history of the lowest organized beings very closely resemble the
first stages in the development of more advanced groups; that as Allen
Thomson has truly observed,[92] "the occurrence of segmentation and the
regularity of its phenomena are so constant that we may regard it as one
of the best established series of facts in organic nature."

It is true that normal yolk-segmentation is not universal in the animal
kingdom; that there are great groups in which the yolk does not divide
in this manner,--perhaps owing to some difference in its relation to the
germinal vesicle, or perhaps because one of the suppressed stages in
embryological development, many examples might be given, not only in
zoology, but, as I may state on the authority of Dr. Hooker, in botany
also. But, however, this may be, it is surely not uninteresting, nor
without significance, to find that changes which constitute the
life-history of the lowest creatures for the initial stages even of the
highest.

Returning, in conclusion, to the immediate subject of this work, I have
pointed out that many beetles and other insects are derived from larvae
closely resembling _Campodea_.

Since, then, individual insects are certainly in many cases developed
from larvae closely resembling the genus _Campodea_, why should it be
regarded as incredible that insects as a group have gone through similar
stages? That the ancestors of beetles under the influence of varying
external conditions, and in the lapse of geological ages, should have
undergone changes which the individual beetle passes through under our
own eyes and in the space of a few days, is surely no wild or
extravagant hypothesis. Again, other insects come from vermiform larvae
much resembling the genus _Lindia_, and it has been also repeatedly shown
that in many particulars the embryo of the more specialized forms
resembles the full-grown representatives of lower types. I conclude,
therefore, that the Insecta generally are descended from ancestors
resembling the existing genus _Campodea_, and that these again have arisen
from others belonging to a type represented more or less closely by the
existing genus _Lindia_.

Of course it may be argued that these facts have not really the
significance which they seem to me to possess. It may be said that when
Divine power created insects, they were created with these remarkable
developmental processes. By such arguments the conclusions of geologists
were long disputed. When God made the rocks, it was tersely said, He
made the fossils in them. No one, I suppose, would now be found to
maintain such a theory; and I believe the time will come when it will be
generally admitted that the structure of the embryo, and its
developmental changes, indicate as truly the course of organic
development in ancient times as the contents of rocks and their sequence
teach us the past history of the earth itself.


FOOTNOTES:

[1] Darwin's "Researches into the Geology and Natural History of the
Countries visited by H.M.S. _Beagle_," p. 326.

[2] Introduction to Entomology, vi. p. 50.

[3] Manual of Entomology, p. 30.

[4] Linnean Journal, vol. xi.

[5] Introduction to the Modern Classification of Insects, p. 17.

[6] Linnean Transactions, 1863--"On the Development of _Chloeon_."

[7] The figures on the first four plates are principally borrowed from
Mr. Westwood's excellent "Introduction to the Modern Classification of
Insects."

[8] "Sur la Domestication des _Clavigers_ par les Fourmis." Bull. de la
Soc. d'Anthropologie de Paris, 1868, p. 315.

[9] Westwood's Introduction, vol. i. p. 36.

[10] Westwood's Introduction, vol. ii. p. 52.

[11] Die Fortpflanzung und Entwickelung der Pupiparen. Von Dr. R.
Leuckart. Halle. 1848.

[12] Ann. des Sci. Nat., ser. 4, tome vii. See also _Natural History
Review_, April 1862.

[13] Ann. and Mag. of Nat. Hist. 1852.

[14] Zeits. fuer Wiss. Zool. 1869.

[15] Transactions of the Linnean Society, 1863.

[16] Lectures on the Anatomy, &c. of the Invertebrate Animals.

[17] Untersuchungen ueber die Entwickelung und den Bau der Gliederthiere,
1854.

[18] Linnean Transactions, vol. xxii. 1858.

[19] "Embryological Studies on Hexapodous Insects." Peabody Academy of
Science. Third Memoir.

[20] Mem. de l'Acad. Imp. des Sci. de St. Petersbourg. 1869.

[21] Observationes de Prima Insectorum Genesi, p. 14.

[22] Mem. de l'Acad. Imp. des Sci. de St. Petersbourg. tome xvi. 1871,
p. 35.

[23] Recherches sur l'Evolution des Araignees.

[24] Philosophical Transactions, 1841.

[25] Monog. of the Gymnoblastic or Tubularian Hydroids. See also Hincks,
British Hydroid Zoophytes. Pl. x.

[26] Loc. cit. p. 315.

[27] Philosophical Transactions, 1859, p. 589.

[28] "Facts for Darwin," Eng. Trans. p. 127.

[29] Rolleston, "Forms of Animal Life," p. 146.

[30] A. Agassiz, "Embryology of the Starfish," p. 25; "Embryology of
Echinoderms." Mem. of Am. Ac. of Arts and Sciences N.S. vol. ix. p. 9.

[31] Ueber die Gattungen der Seeigellarven. Siebente Abhandlung. Koen.
Akad. d. Wiss. zu Berlin. Von Joh. Mueller, 1855, Pl. iii. fig. 3.

[32] Huxley, Introduction to the Classification of Animals, p. 45.

[33] Philosophical Transactions, 1865 and 1866.

[34] Loc. cit. Zweit. Abh. Pl. i., figs. 8 and 9.

[35] Thomson, on the Embryology of the Echinodermata, _Natural History
Review_, 1863, p. 415. See also Agassiz, "Embryology of the Starfish,"
p. 62.

[36] A. Agassiz, Embryology of Echinoderms, p. 18.

[37] Hincks. British Hydroid Zoophytes, pp. 120-147.

[38] Zeits. fuer Wiss. Zool. 1864, p. 228.

[39] Introduction to Entomology, 6th ed. vol. i. p. 61.

[40] Metamorphoses de l'Homme et des Animaux, p. 133. See also
Carpenter, Principles of Physiology. 1851, p. 389.

[41] Darwin, Origin of Species, 4th ed. p. 532.

[42] Principles of Biology, vi. p. 349.

[43] For differences in larva consequent on variation in the external
condition, see _ante_, p. 61.

[44] See Hincks. British Hydroid Zoophytes, P. lxii. Agassiz, Sea-side
Studies, p. 43.

[45] See Newport, Phil. Trans., 1832.

[46] Linnean Transactions, 1862.

[47] Origin of Species, 4th ed., pp. 14 and 97.

[48] On the Alternation of Generations. By J. J. Steenstrup. Trans. by
C. Busk, Esq. Ray Society. 1842.

[49] Zeit. fuer Wiss. Zool. 1863.

[50] Mem. de l'Acad. Imp. de St. Petersbourg. vol. xv. 1870.

[51] Of course all animals in which the sexes are distinct are in one
sense dimorphic.

[52] "There is no such thing as a true case of 'alternation of
generations in the animal kingdom;' there is only an alternation of true
generation with the totally distinct process of gemmation or
fission."--HUXLEY _on Animal Individuality_, Ann. and Mag. of Nat. Hist.
June 1852.

[53] Prince Hohenstiel Schwangau, p. 68.

[54] Journal of the Royal Institution. April 1873.

[55] "Embryology of Echinoderms," l. c. p. 15.

[56] Mr. and Mrs. Agassiz: "Sea-side Studies," p. 139.

[57] l. c. p. 138.

[58] Wien. Zool. Bot. Gesells, 1869.

[59] Linnean Transactions, 1863.

[60] Linnean Transactions, 1866, vol. xxv.

[61] Linnean Transactions, vol. xxiv. p. 65.

[62] Siebold und Kolliker's Zeitschr. f. Wiss. Zool., 1864.

[63] Linnean Journal, vol. xi.

[64] Facts for Darwin, p. 120.

[65] A still nearer approach is afforded by the genus _Peripatus_, which
since the above was written has been carefully described, especially by
Moseley and Hutton. There are several species, scattered over the
southern hemisphere. In general appearance they look like a link between
a caterpillar and a centipede. They have a pair of antennae, two pairs of
jaws, and (according to the species) from fourteen to thirty-three pairs
of legs. They breathe by means of tracheae, which open diffusely all over
the body.

[66] Unters. ueb. die Entwick, und den Bau der Gliederthiere, p. 73.

[67] Linnean Transactions, v. xxii.

[68] Facts for Darwin, trans. by Dallas, p. 118. See also Darwin,
"Origin of Species," p. 530. 4th ed.

[69] Mem. Peabody Academy of Science, v. I. No, 3.

[70] Wien. Zool. Bott. Gesells. 1869, p. 310.

[71] See also the descriptions given by Dujardin (Ann. des Sci. Nat.
1851, v. xv.) and Claparede (Anat. und Entwickl. der Wirbel osen Thiere)
of the interesting genus _Echinoderes_, which these two eminent
naturalists unite in regarding as intermediate between the Annelides and
the Crustacea.

[72] "On a New Rotifer." _Monthly Microscopical Journal_, Sept. 1871.

[73] Generelle Morphologie, vol. ii. p. 79.

[74] Monographie der Moneren, p. 43.

[75] Gegenbaur. Grund. d. Vergleich. Anat. p. 210. See also Dr. M. S.
Schultze, Beitraege zur Naturg. der. Turbellarien. 1851. Pl. vi. fig. 1.

[76] Monographieder Moneren, p. 10.

[77] See Kauffmann, Ueber die Entwickelung and systematische Stellung
der Tardigraden. Zeits. f. Wiss. Zool. 1851, p. 220.

[78] It is true that among the Insecta generally the first stages of
development differ in appearance considerably from those above
described; those of _Platygaster_, as figured by Ganin (ante Figs. 17-22),
being very exceptional.

[79] Monograph of the Gymnoblastic or Tubularian Hydroids, by G. J.
Allman, Ray Soc. 1871, p. 86.

[80] Mem. sur les Vers Intestinaux, 1858.

[81] Natural History Review, 1861, p. 44.

[82] Ann. des Sci. Nat. 1847, p. 90.

[83] Fauna littoralis Norvegiae, pl. viii.

[84] Trans. of the Microsc. Soc. of London, 1851.

[85] Quarterly Journal of Microsc. Science, 1853.

[86] Wiegmann's Archiv., 1840, p. 196.

[87] Ueber die Erzeugung von Schnecken in Holothurier. Berlin, Bericht,
1851. Ann. Nat. Hist. 1852, v. ix. Mueller's Archiv., 1852.

[88] Natuerliche Schoepfungsgeschichte, pl. x.

[89] Ann. des Sci. Nat. 1853, p. 89.

[90] Ann. des Sci. Nat. 1857, pl. vi.

[91] Cyclopaedia of Anatomy and Physiology. Art. Ovum, p. 4.

[92] Thomson, loc. cit. Article, Ovum, p. 139.


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NATURE contains Original Articles on all subjects coming within the
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