On the Origin and Metamorphoses of Insects Part 4

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 Kolliker,[21] it would appear that in the Coleopterous genus _Donacia_ the segments and appendages appear simultaneously.

Kolliker 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 h.o.m.omorphous 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.

[Ill.u.s.tration: 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 a.n.a.l et le capuchon cephalique."

[Ill.u.s.tration: 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 sh.e.l.l, and exposure of the embryo within it, which, however, remains for some time longer in connection with the sh.e.l.l, 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 h.o.m.omorphous insects-which, as we have seen, is now so short, and pa.s.sed 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 ill.u.s.trations of past history, but without direct advantage under present conditions, are rapidly pa.s.sed through, and even, as it would appear, in some cases altogether omitted.

[Ill.u.s.tration: 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 unsurpa.s.sed 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."

[Ill.u.s.tration: FIG. 35.-Portion of colony of _Bougainvillea fruticosa_, more magnified.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 37, Larva of Prawn, Nauplius stage (after F.

Muller). 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. Muller 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 princ.i.p.ally effected through the walls of the carapace. The abdomen and tail are dest.i.tute 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, pa.s.s through a stage more or less resembling that shown in Fig. 38. On the other hand, the great group of Edriopthalma, comprising Amphipoda (sh.o.r.e-hoppers, &c.) and Isopoda (wood-lice, &c.) pa.s.s 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. Muller,[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 ma.s.s 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] Muller 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. Aga.s.siz, 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 Muller.[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 ca.n.a.l; 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 ma.s.s or store of what is called blastema, or the raw material of the animal body. This blastema then begins to change, and gradually a.s.sumes the form of the perfect Echinoderm.[32]

[Ill.u.s.tration: FIG. 39.-Larva of _Echino cidaris_, seen from above 6/10 (after Muller).]

[Ill.u.s.tration: FIG. 40, Larva of _Echinus_, 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'_, sophagus; _b_, stomach; _b'_, intestine; _o_, posterior orifice; _d_, ciliated bands; _f_, ciliated epaulets; _c_, disc of future _Echinus_ (after Muller).]

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 sophagus, _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.

[Ill.u.s.tration: FIG. 41.-_Comatula rosacea_ (after Forbes).]

[Ill.u.s.tration: 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 k.n.o.b 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 Muller[34] near Ma.r.s.eilles. _a_ is the mouth, _b_ the sophagus, _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.

[Ill.u.s.tration: FIG. 45, Larva of Starfish (Bipinnaria), 100 (after Muller). 46, Larva of Starfish (Bipinnaria), 100, seen from the side-_a_, mouth; _b_, sophagus; _c_, stomach; _c'_, intestine. 47, Larva of another Bipinnaria, showing the commencement of the Starfish-_g_, ca.n.a.l 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 pa.s.sing through metamorphoses, while others, as for instance _Ophiopholis bellis_, "is developed very much after the method of _Asteracanthion Mulleri_, without pa.s.sing 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 pa.s.s 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 a.s.signed 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 pa.s.ses 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 coloured when young. The young of the lion and the puma are often striped, and the ftal 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 pa.s.sed through within the egg, and thus do not come within the popular ken. "La larve," says, Quatref.a.ges, "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 s.p.a.ce 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 Muller 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 Muller 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 cla.s.s, order, &c., is only afterwards developed." Aga.s.siz 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 pa.s.sed 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 circ.u.mstances 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.