We may, however, as bearing upon this difficult question, cite some remarkable discoveries of Professor Ganin, a Russian naturalist, on the early stages of certain ichneumon parasites, which show some worm features in their embryonic development. In a species of Platygaster (Fig. 192, P. error of Fitch), which is a parasite on a two-winged gall fly, the earliest stage observed after the egg is laid is that in which the egg contains a single cell with a nucleus and nucleolus. Out of this cell (Fig. 193 _A_, _a_) arise two other cells. The central cell (_a_) gives origin to the embryo. The two outer ones multiply by subdivision and form the embryonal membrane, or "amnion," which is a provisional envelope and does not a.s.sist in building up the body of the germ. The central single cell, however, multiplies by the subdivision of its nucleus, thus building up the body of the germ. Figure 193 _B_, _g_, shows the yolk or germ just forming out of the nuclei (_a_) and _b_, the peripheral cells of the blastoderm skin, or "amnion." Figure 193 _C_ shows the yolk transformed into the embryo (_g_), with the outer layer of blastodermic cells (_b_). The body of the germ is infolded, so that the embryo appears bent on itself. Figure 193 _D_ shows the embryo much farther advanced, with the two pairs of lobes (_md_, rudimentary mandibles; _d_, rudimentary pad-like organs, seen in a more advanced stage in _E_), and the bilobate tail (_st_). Figure 194 (_m_, mouth; _at_, rudimentary antennae; _md_, mandibles; _d_, tongue-like appendages; _st_, a.n.a.l stylets; the subject of this figure is of a different species from the insect previously figured, which, however, it closely resembles) shows the first larva stage after leaving the egg. This strange form, the author remarks, would scarcely be thought an insect, were not its origin and farther development known, but rather a parasitic Copepodous crustacean, whence he calls this the Cyclops-like stage. In this condition it clings to the inside of its host by means of its hook-like jaws (_md_), moving about like a Cestodes embryo with its well known six hooks. The tail moves up and down, and is of but little a.s.sistance in its efforts to change its place. Singularly enough, the nervous, vascular, and respiratory systems (tracheae) are wanting, and the alimentary ca.n.a.l is a blind sac, remaining in an indifferent, or unorganized state. How long it remains in this state could not be ascertained.
[Ill.u.s.tration: 193. Development of Platygaster.]
[Ill.u.s.tration: 194. First Larva of Platygaster.]
[Ill.u.s.tration: 195. Second Larva of Platygaster.]
The second larval stage (Fig. 195; _oe_, oesophagus; _ng_, supra-oesophageal ganglion; _n_, nervous cord; _ga_, and _g_, genital organs; _ms_, band of muscles) is attained by means of a moult, as usual in the metamorphoses of insects. With the change of skin the larva entirely changes its form. So-called hypodermic cells are developed. The singular tail is dropped, the segments of the body disappear, and the body grows oval, while within begins a series of remarkable changes, like the ordinary development of the embryo of most other insects within the egg. The cells of the hypodermis multiply greatly, and lie one above the other in numerous layers. They give rise to a special primitive organ closely resembling the "primitive band" of all insect embryos. The alimentary ca.n.a.l is made anew, and the nervous and vascular systems now appear, but the tracheae are not yet formed. It remains in this state for a much longer period than in the previous stage.
[Ill.u.s.tration: 196. Third Larva of Polynema.]
The third larval form only a few live to reach. This is of the usual long, oval form of the larvae of the ichneumons, and the body has thirteen segments exclusive of the head. The muscular system has greatly developed and the larva is much more lively in its motions than before.
The new organs that develop are the air tubes and fat bodies. The "imaginal disks" or rudimentary portions destined to develop and form the skin of the adult, or imago, arise in the pupa state, which resembles that of other ichneumons. These disks are only engaged, in Platygaster, in building up the rudimentary appendages, while in the flies (Muscidae and Corethra) they build up the whole body, according to the remarkable discovery of Weismann.
Not less interesting is the history of the development of a species of Polynema, another egg-parasite, which lays its eggs (one, seldom two) in the eggs of a small dragon fly, Agrion virgo, which oviposits in the parenchyma of the leaves of waterlilies. The eggs develop as in Platygaster. The earliest stage of the embryo is very remarkable. It leaves the egg when very small and immovable, and with scarcely a trace of organization, being a mere flask-shaped sac of cells.[23] It remains in this state five or six days.
In the second stage, or Histriobdella-like form, the larva is, in its general appearance, like the low worm to which Ganin compares it. It may be described as bearing a general resemblance to the third and fully developed larval form (Fig. 196, _tg_, three pairs of abdominal tubercles destined to form the sting; _l_, rudiments of the legs; _fk_, portion of the fatty body; _at_, rudiments of the antennae; _fl_, imaginal disks, or rudiments of the wings). No tracheae are developed in the larva, nor do any exist in the imago. (Ganin thinks, that as these insects are somewhat aquatic, the adult insects flying over the surface of the water, the wings may act as respiratory organs, like gills.) It lives six to seven days before pupating, and remains from ten to twelve days in the pupa state.
The origin of the sting is clearly ascertained. Ganin shows that it consists of three pairs of tubercles, situated respectively on the seventh, eighth, and ninth segments of the abdomen (Fig. 196, _tg_). The labium is not developed from a pair of tubercles, as is usual, but at once appears as an unpaired, or single organ. The pupa state lasts for five or six days, and when the imago appears it eats its way through a small round opening in the end of the skin of its host, the Agrion larva.
[Ill.u.s.tration: 197. Development of Egg-parasites.]
The development of Ophloneurus, another egg-parasite, agrees with that of Platygaster and Polynema. This egg-parasite pa.s.ses its early life in the eggs of Pieris bra.s.sicae, and two or three live to reach the imago state, though about six eggs are deposited by the female. The eggs are oval, and not stalked. The larva is at first of the form indicated by figure 197 _E_, and when fully grown becomes of a broad oval form, the body not being divided into segments. It differs from the genera already mentioned, in remaining within its egg membrane, and not a.s.suming their strange forms. From the non-segmented, sac-like larva, it pa.s.ses directly into the pupa state.
The last egg-parasite noticed by Ganin, is Teleas, whose development resembles that of Platygaster. It is a parasite in the eggs of Gerris, the Water Boatman. Figure 197 _A_ represents the egg; _B_, _C_, and _D_, the first stage of the larva, the abdomen (or posterior division of the body) being furnished with a series of bristles on each side. (_B_ represents the ventral, _C_ the dorsal, and _D_ the profile view; _at_, antennae; _md_, hook-like mandibles; _mo_, mouth; _b_, bristles; _m_, intestine; _sw_, the tail; _ul_, under lip or labium.) In the second larval stage, which is oval in form, and not segmented, the primitive band is formed.
In concluding the account of his remarkable discoveries, Ganin draws attention to the great differences in the formation of the eggs and the germs of these parasites from what occurs in other insects. The egg has no nutritive cells; the formation of the primitive band, usually the first indication of the germ, is r.e.t.a.r.ded till the second larval stage is attained; and the embryonal membrane is not h.o.m.ologous with the so-called "amnion" of other insects, but may possibly be compared with the skin developed on the upper side of the low, worm-like acarian, Pentastomum, and the "larval skin" of the embryos of many low Crustacea.
He says, also, that we cannot, perhaps, find the h.o.m.ologues of the provisional organs of the larvae, such as the singularly shaped antennae, the claw-like mandibles, the tongue-or ear-like appendages, in other Arthropoda (insects and Crustacea); but that they may be found in the parasitic Lernaean crustaceans, and in the leeches, such as Histriobella.
He is also struck by the similarity in the development of these egg-parasites to that of a kind of leech (Nephelis), the embryo of which is provided with ciliae, recalling the larva of Teleas (Fig. 197 _B_, _C_), while in the true leeches (Hirudo) the primitive band is not developed until after they have pa.s.sed through a provisional larval stage.
This complicated metamorphosis of the egg-parasites, Ganin also compares to the so-called "hyper-metamorphosis" of certain insects (Meloe, Sitaris, and the Stylopidae) made known by Siebold, Newport and Fabre, and he considers it to be of the same nature.
He also, in closing, compares such early larval forms as those given in figures 193 _E_ and 194, to the free swimming Copepoda. Finally, he says a few words on the theory of evolution, and remarks "there is no doubt that, if a solution of the questions arising concerning the genealogical relations of different animals among themselves is possible, comparative embryology will afford the first and truest principles." He modestly suggests that the facts presented in his paper will widen our views on the genetic relations of the insects to other animals, and refers to the opinion first expressed by Fritz Muller (Fur Darwin, p. 91), and endorsed by Haeckel in his "Generelle Morphologie," that we must seek for the ancestors of insects and Arachnida in the Zoea form of Crustacea. He cautiously remarks, however, that "the embryos and larvae observed by me in the egg-parasites open up a new and wide field for a whole series of such considerations; but I will suppress them, since I am firmly convinced that a theory, which I build up to-day, can easily be destroyed with some few facts which I learn to-morrow. Since comparative embryology as a science does not yet exist, so do I think that all genetic theories are too premature, and without a strong scientific foundation."
The writer is perhaps less cautious, but he cannot refrain from making some reflections suggested by the remarkable discoveries of Ganin. In the first place, these facts bear strongly on the theory of evolution by "acceleration and r.e.t.a.r.dation." In the history of these early larval stages we see a remarkable acceleration in the growth of the embryo. A simple sac of unorganized cells, with a half-made intestine, so to speak, is hatched, and made to perform the duty of an ordinary, quite highly organized larva. Even the formation of the "primitive band,"
usually the first indication of the organization of the germ, is postponed to a comparatively late period in larval life. The different anatomical systems, _i.e._, the heart with its vessels, the nervous system and the respiratory system (tracheae), appear at longer or shorter intervals, while in one genus the tracheae are not developed at all. Thus some portions of the animal are accelerated in their development more than others, while others are r.e.t.a.r.ded, and in some species certain organs are not developed at all. Meanwhile all live in a fluid medium, with much the same habits, and surrounded with quite similar physical conditions.
The highest degree of acceleration is seen in the reproductive organs of the Cecidomyian larva of Miastor, which produces a summer brood of young, alive, and living free in the body of the child-parent; and in the pupa of Chironomus, which has been recently shown by Von Grimm, a fellow countryman of Ganin, to produce young in the spring, while the adult fly lays eggs in the autumn in the usual manner. This is in fact a true virgin reproduction, and directly comparable to the alternation of generations observed in the jelly fishes, in Salpa, and certain intestinal worms. We can now, in the light of the researches of Siebold, Leuckart, Ganin and others, trace more closely than ever the connection between simple growth and metamorphosis, and metamorphosis and parthenogenesis, and perceive that they are but the terms of a single series. By the acceleration in the development of a single set of organs (the reproductive), no more wonderful than the acceleration and r.e.t.a.r.dation of the other systems of organs, so clearly pointed out in the embryos of Platygaster and its allies, we see how parthenogenesis under certain conditions may result. The barren Platygaster larva, the fertile Cecidomyia larva, the fertile Aphis larva, the fertile Chironomus pupa, the fertile hydroid polype, and the fertile adult queen bee are simply animals in different degrees of organization, and with reproductive systems differing not in quality, but in the greater or less rapidity of their development as compared with the rest of the body.
Another interesting point is, that while the larvae vary so remarkably in form, the adult ichneumon flies are remarkably similar to one another.
Do the differences in their larval history seem to point back to certain still more divergent ancestral forms?
These remarkable hyper-metamorphoses remind us of the metamorphosis of the embryo of Echinoderms into the Pluteus-and Bipinnaria-forms of the starfish, sea urchins and Holothurians;[24] of the Actinotrocha-form larva of the Sipunculoid worms; of the Tornaria into Balanoglossus, the worm; of the Cercaria-form larva of Distoma; of the Pilidium-form larva of Nemertes; and the larval forms of the leeches;[25] as well as the mite Pentastomum, and certain other aberrant mites, such as Myobia.
While Fritz Muller and Dohrn have considered the insects as having descended from the Crustacea (some primitive zoea-form), and Dohrn has adduced the supposed zoea-form larva of these egg-parasites as a proof, we cannot but think, in a subject so purely speculative as the ancestry of animals, that the facts brought out by Ganin tend to confirm our theory, that the ancestry of all the insects (including the Arachnids and Myriopods) should be traced directly to the worms. The development of the degraded, aberrant Arachnidan Pentastomum accords, in some important respects, with that of the intestinal worms. The Leptus-form larva of Julus, with its strange embryological development, in some respects so like that of some worms, points in that direction, as certainly as does the embryological development of the egg-parasite Ophioneurus. The Nauplius form of the embryo or larva of nearly all Crustacea, also points back to the worms as their ancestors, the divergence having perhaps originated, as we have suggested, in the Rotatoria.
While the Crustacea may have resulted from a series of prototypes leading up from the Rotifers (Fig. 198), it is barely possible that one of these creatures may have given rise to a form resulting in two series of beings, one leading to the Leptus form, the other to the Nauplius.
For the true Annelides (Chaetopods) are too circ.u.mscribed and h.o.m.ogeneous a group to allow us to look to them for the ancestral forms of insects.
But that the insects may have descended from some low worms is not improbable when we reflect that the Syllis and allied genera of Annelides bear appendages consisting of numerous joints; indeed, the strange Dujardinia rotifers, figured by Quatref.a.ges, in its general form is remarkably like the larva of Chloeon. It has a quite distinct head, bearing five long, slender, jointed antennae, and but eight or nine rings to the body, which ends in two long, many jointed appendages exactly like the tentacles. Quatref.a.ges adds, that its movements are usually slow, but "when it wishes to move more rapidly, it moves its body alternately up and down with much vivacity, and shoots forwards by bounds, so to speak, a little after the manner of the larvae of the mosquito" (Histoire Naturelle des Anneles, Tome 2, p. 69). The gills of aquatic insects only differ from those of worms in possessing tracheae, though the gills of the Crustacea may be directly compared with those of insects.
[Ill.u.s.tration: 198. A Rotifer.]
But when once inside the circle of the cla.s.s of insects the ground is firmer, as our knowledge is surer. Granting now that the Leptus-like ancestor of the six-footed insects has become established, it is not so difficult to see how the Podurae and finally a form like Campodea appeared. Aquatic forms resembling the larva of the Ephemerae, Perlae and, more remotely, the Forficulae and white ants of to-day were probably evolved with comparative suddenness. Given the evolution of forms like the earwigs (Forficula), c.o.c.kroaches and white ants (Termes), the latter of which abounded in the coal period, and it was not a great step forward to the evolution of the Dragonflies, the Psocus, the Chrysopa, the lice or parasitic Hemiptera, together with Thrips, thus forming the establishment of lines of development leading up to those Neuroptera with a complete metamorphosis, and finally to the gra.s.shoppers and other forms of Orthoptera, together with the Hemiptera.
[Ill.u.s.tration: 199. Chrysopa.]
[Ill.u.s.tration: 200. Panorpa.]
We have thus advanced from wingless to winged forms, _i. e._, from insects without a metamorphosis to those with a partial metamorphosis like the Perlas; to the May flies and Dragon flies, in which the adult is still more unlike the larva; to the Chrysopa (Fig. 199) and Forceps Tails (Panorpa, Fig. 200) and Caddis flies, in which, especially the latter, the metamorphosis is complete, the pupa being inactive and enclosed in a coc.o.o.n.
[Ill.u.s.tration: 201. Embryo of Diplax.]
Having a.s.sumed the creation of our Leptus by evolutional laws, we must now account for the appearance of tracheae and those organs so dependent on them, the wings, which, by their presence and consequent changes in the structure of the crust of the body, afford such distinctive characters to the flying insects, and raise them so far above the creeping spiders and centipedes. Our Leptus at first undoubtedly breathed through the skin, as do most of the Poduras, since we have been unable to find tracheae in them, nor even in the prolarva of a genus of minute ichneumon egg parasites, nor in the Linguatulae and Tardigrades, and some mites, such as the Itch insect and the Demodex, and other Acari. In the Myriopod, Pauropus, Lubbock was unable to find any traces of tracheae. If we examine the embryo of an insect shortly before birth, as in the young Dragon fly (figure 201, the dotted line _t_ crosses the rudimentary tracheae), we find it to consist of two simple tubes with few branches, while there are no stigmata, or breathing holes, to be seen in the sides of the body. This fact sustains the view of Gegenbaur[26] that at first the tracheae formed two simple tubes in the body-cavity, and that the primary office of these tubes was for lightening the body, and that their function as respiratory tubes was a secondary one. The aquatic Protoleptus, as we may term the ancestor of Leptus, may have had such tubes as these, which acted like the swimming bladder of fishes for lightening the body, as suggested by Gegenbaur. It is known that the swimming bladder of fishes becomes developed into the lungs of air-breathing vertebrates and man himself. As our Leptus adopted a terrestrial life and needed more air, a connection was probably formed by a minute branch on each side of the body with some minute pore (for such exist, whose uses are as yet unknown) through the skin, which finally became specialized into a stigma, or breathing pore; and from the tracheal system being closed, we now have the open tracheal system of land insects.
The next inquiry is as to the origin of the wings. Here the question arises if wingless forms are exceptional among the winged insects, and the loss of wings is obviously dependent on the habits (as in the lice), and environment of the species (as in beetles living on islands, which are apt to lose the hinder pair of wings), why may not their acquisition in the first place have been due to external agencies; and, as they are suddenly discarded, why may they not have suddenly appeared in the first place? In aquatic larvae there are often external gill-like organs, being simple sacs permeated by tracheae (as in Agrion, Fig. 129, or the May flies). These organs are virtually aquatic wings, aiding the insect in progression as well as in aerating the blood, as in the true wings. They are very variable in position, some being developed at the extremity of the abdomen, as in Agrion, or along the sides, as in the May flies, or filiform and arranged in tufts on the under side of the body, as in Perla; and the naturalist is not surprised to find them absent or present in accordance with the varying habits of the animal. For example, in the larvae of the larger Dragon flies (Libellula, etc.) they are wanting, while in Agrion and its allies they are present.
Now we conceive that wings formed in much the same way, and with no more disturbance, so to speak, to the insect"s organization, appeared during a certain critical period in the metamorphosis of some early insect. As soon as this novel mode of locomotion became established we can easily see how surrounding circ.u.mstances would favor their farther development until the presence of wings became universal. If s.p.a.ce permitted us to pursue this interesting subject farther, we could show how invariably correlated in form and structure are the wings of insects to the varied conditions by which they are surrounded, and which we are forced to believe stand in the relation of cause to effect. Again, why should the wings always appear on the thorax and on the upper instead of the under side? As this is the seat of the centre of gravity, it is evident that cosmical laws as well as the more immediate laws of biology determine the position and nature of the wings of an insect.
Correlated with the presence of wings is the wonderful differentiation of the crust, especially of the thorax, where each segment consists of a number of distinct pieces; while in the spiders and Myriopods the segments are as simple as in the abdominal segments of the winged insect. It is not difficult here to trace a series leading up from the Poduras, in which the segments are like those of spiders, to the wonderful complexity of the parts in the thoracic segments of the Lepidoptera and Hymenoptera.
In his remarks "On the Origin of Insects,"[27] Sir John Lubbock says, "I feel great difficulty in conceiving by what natural process an insect with a suctorial mouth like that of a gnat or b.u.t.terfly could be developed from a powerfully mandibulate type like the Orthoptera, or even from the Neuroptera." Is it not more difficult to account for the origin of the mouth-parts at all? They are developed as tubercles or folds in the tegument, and are h.o.m.ologous with the legs. Figure 186 shows that the two sorts of limbs are at one time identical in form and relative position. The thought suggests itself that these long, soft, finger-like appendages may have been derived from the tentacles of the higher worms, but the grounds for this opinion are uncertain. At any rate, the earliest form of limb must have been that of a soft tubercle armed with one, or two, or many terminal claws, as seen in aquatic larvae, such as Chironomus (Fig. 202), Ephydra (Fig. 203 _a_, _b_, _c_, pupa) and many others. As the Protoleptus a.s.sumed a terrestrial life and needed to walk, the rudimentary feet would tend to elongate, and in consequence need the presence of chitine to harden the integument, until the habit of walking becoming fixed, the necessity of a jointed structure arose. After this the different needs of the offspring of such an insect, with their different modes of taking food, vegetable or animal, would induce the diverse forms of simple, or raptorial, or leaping or digging limbs. A peculiar use of the anterior members, as seen in grasping the food and conveying it to the mouth (perhaps originally a simple orifice with soft lips, as in Peripatus), would tend to cause such limbs to be grouped together, to concentrate around the mouth-opening, and to be directed constantly forwards. With use, as in the case of legs, these originally soft mouth-feet would gradually harden at the extremities, until serviceable in biting, when they would become jaws and palpi. Given a mouth and limbs surrounding it, and we at once have a rude head set off from the rest of the body. And in fact such is the history of the development of these parts in the embryo. At first the head is indicated by the buds forming the rudiments of limbs; the segments to which they are attached do not form a true head until after the mouth-parts have attained their jaw-like characters, and it is not until the insect is about to be hatched, that the head is definitely walled in.
[Ill.u.s.tration: 202. Foot of Chironomus.]
[Ill.u.s.tration: 203. Ephydra.]
We have arrived, then, at our Leptus, with a head bearing two pairs of jaws. The spiders and mites do not advance beyond this stage. But in the true insects and Myriopods, we have the addition of special sense organs, the antennae, and another pair of appendages, the l.a.b.i.al palpi.
It is evident that in the ancestor of these two groups the first pair of appendages became early adapted for purely sensory purposes, and were naturally projected far in advance of the mouth, forming the antennae.
Before considering the changes from the mandibulate form of insects to those with mouth parts adapted for piercing and sucking, we must endeavor to learn how far it was possible for the caterpillar or maggot to become evolved from the Leptus-like larvae of the Neuroptera, Orthoptera, Hemiptera and most Coleoptera. I may quote from a previous article[28] a few words in relation to two kinds of larvae most prevalent among insects. "There are two forms of insectean larvae which are pretty constant. One we call _leptiform_, from its general resemblance to the larvae of the mites (Leptus). The larvae of all the Neuroptera, except those of the Phryganeidae and Panorpidae (which are cylindrical and resemble caterpillars), are more or less leptiform, _i. e._, have a flattened or oval body, with large thoracic legs. Such are the larvae of the Orthoptera and Hemiptera, and the Coleoptera (except the Curculionidae; possibly the Cerambycidae and Buprestidae, which approach the maggot-like form of the larvae of weevils). On the other hand, taking the caterpillar or bee larva, with their cylindrical, fleshy bodies, in most respects typical of larval forms of the Hymenoptera, Lepidoptera and Diptera, as the type of the _cruciform_ larva, etc. * * * The larvae of the earliest insects were probably leptiform, and the cruciform condition is consequently an acquired one, as suggested by Fritz Muller."[29] It seems that these two sorts of larvae had also been distinguished by Dr. Brauer in the article already referred to, with which, however, the writer was unacquainted at the time of writing the above quoted article. The similar views presented may seem to indicate that they are founded in nature. Dr. Brauer, after remarking that the Podurids seemed to fulfil Haeckel"s idea of what were the most primitive insects, and noticing how closely they resemble the larvae of Myriopods, says, "specially interesting are those forms among the Poduridae which are described as Campodea and j.a.pyx, since the larvae of a great number of insects may be traced back to them"; but he adds, and with this view we are unable to agree, "while others, the caterpillar-like forms (Raupenform), resulted from them by a retrograde process, and also the still lower maggot-like forms. While on the one hand Campodea, with its abdominal feet, and the larva of Lithobius are related, so on the other the Lepismatidae, which are very near the Blattariae, are nearly related to the Myriopods, since their abdominal segments often bear appendages (Machilis). The Campodea-form appears in most of the Pseudoneuroptera [Libellulids, Ephemerids, Perlids, Psocids and Termes], Orthoptera, Coleoptera, Neuroptera, perhaps modified in the Strepsiptera [Stylops and Xenos] and Coccidae in their first stage of development, and indeed in many of these at their first moult." Farther on he says, "A larger part of the most highly developed insects a.s.sume another larva-form, which appears not only as a later acquisition, through accommodation with certain definite relations, but also arises as such before our eyes. The larvae of b.u.t.terflies and moths, of saw flies and Panorpae, show the form most distinctly, and I call this the caterpillar form (Raupenform). That this is not the primitive form, but one later acquired, we see in the beetles. The larvae of Meloe and Sitaris in their fully grown condition possess the caterpillar form, but the new born larvae of these genera show the Campodea form. The last form is lost as soon as the larva begins its parasitic mode of life. * * * The larger part of the beetles, the Neuroptera in part, the bees and flies (the last with the most degraded maggot form) possess larvae of this second form." He considers that the caterpillar form is a degraded Campodea form, the result of its stationary life in plants or in wood.
[Ill.u.s.tration: Pl 2. EXAMPLES OF LEPTIFORM LARVae.
EXPLANATION OF PLATE 2. Figure 1, different forms of Leptus; 2, Diplax; 3, Coccinella larva; 4, Cicada larva; 5, Cicindela larva; 6, Ant Lion; 7, Calligrapha larva; 8, Aphis larva; 9, Hemerobius larva; 10, Glyrinua larva; 11, Carabid larva; 12, Meloe larva.]
[Ill.u.s.tration: Pl 3. EXAMPLES OF ERUCIFORM LARVae.
EXPLANATION OF PLATE 3. Figure 1. Panorpa larva; 2, Phryganea larva; 3, Weevil larva; 4, third larva of Meloe; 5, Chionea larva; 6, Carpet Worm; 7, Phora larva; 8, Wheat Caterpillar; 9, Sphinx Caterpillar; 10, Acronycta? larva; 11, Saw Fly larva; 12, Abia Saw Fly larva; 13, Halictus larva; 14, Andrena larva.]
[Ill.u.s.tration: 204. Tipula Larva.]
For reasons which we will not pause here to discuss, we have always regarded the eruciform type of larva as the highest. That it is the result of degradation from the Leptus or Campodea form, we should be unwilling to admit, though the maggots of flies have perhaps retrograded from such forms as the larvae of the mosquitoes and crane flies (Tipulids, Fig. 204).
That the cylindrical form of the bee grub and caterpillar is the result of modification through descent is evident in the caterpillar-like form of the immature Caddis fly (Pl. 3, fig. 2). Here the fundamental characters of the larva are those of the Corydalus and Sialis and Panorpa, types of closely allied groups. The features that remind us of caterpillars are superadded, evidently the result of the peculiar tube-inhabiting habits of the young Caddis fly. In like manner the caterpillar-form is probably the result of the leaf-eating life of a primitive Leptiform larva. In like manner the soft-bodied maggot of the weevil is evidently the result of its living habitually in cavities in nuts and fruits. Did the soft, baggy female Stylops live exposed, like its allies in other families, to an out-of-doors life, its skin would inevitably become hard and chitinous. In these and mult.i.tudes of other cases the adaptation of the form of the insect to its mode of life is one of cause and effect, and not a bit less wonderful after we know what induced the change of form.
Having endeavored to show that the caterpillar is a later production than the young, wingless c.o.c.kroach, with which geological facts harmonize, we have next to account for the origin of a metamorphosis in insects. Here it is necessary to disabuse the reader"s mind of the prevalent belief that the terms larva, pupa and imago are fixed and absolute. If we examine at a certain season the nest of a humble bee, we shall find the occupants in every stage of growth from the egg to the pupa, and even to the perfectly formed bee ready to break out of its larval cell. So slight are the differences between the different stages that it is difficult to say where the larval stage ends and the pupa begins, so also where the pupal state ends and the imago begins. The following figures (205-208) will show four of the most characteristic stages of growth, but it should be remembered that there are intermediate stages between. Now we have noticed similar stages in the growth of a moth, though a portion of them are concealed beneath the hard, dense chrysalis skin. The external differences between the larval and pupal states are fixed for a large part of the year in most b.u.t.terflies and moths, though even in this respect there is every possible variation, some moths or b.u.t.terflies pa.s.sing through their transformations in a few weeks, others requiring several months, while still others take a year, the majority of the moths living under ground in the pupa state for eight or nine months. The stages of metamorphosis in the Diptera are no more suddenly acquired than in the bee or b.u.t.terfly. In all these insects the rudiments of the wings, legs, and even of the ovipositor of the adult exist in the young larva. We have found somewhat similar intermediate stages in the metamorphoses of the beetles. The insects we have mentioned are those with a "complete metamorphosis." We have seen that even in them the term "complete" is a relative and not absolute expression, and that the terms larva and pupa are convenient designations for states varying in duration, and a.s.sumed to fulfil certain ends of existence, and even then dependent on length of seasons, variation in climate, and even on the locality. When we descend to the insects with an "incomplete" metamorphosis, as in the May fly, we find that, as in the case of Chloeon, Sir John Lubbock has described twenty-one stages of existence, and let him who can say where the larval ends and the pupal or imaginal stages begin. So in a stronger sense with the gra.s.shopper and c.o.c.kroach. The adult state in these insects is attained after a number of moults of the skin, during each of which the insect gradually draws nearer to the final winged form. But even the so-called pupae, or half winged individuals known not to be adult, in some cases feel the s.e.xual impulse, while a number of species in each of the families represented by these two insects never acquire wings.
[Ill.u.s.tration: 205. Larva. 206. Semi-pupa.