Figs. 23-27Fig. 23, Larva ofPlatygaster(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 ofPlatygaster. The letters indicate the same parts as in the preceding figure. 25, Larva of a third species ofPlatygaster. The letters indicate the same parts as in the preceding figures. 26, Larva ofPlatygasterin the second stage—mo, mouth;slkf, œsophagus;gsae, supra-œsophageal ganglion;lm, muscles;bsm, nervous system;ga,gh, rudiments of the reproductive glands. 27, Larva ofPlatygasterin the third stage—mo, mouth;md, mandibles;gsae, supra-œsophageal ganglion;slk, œsophagus;ag, ducts of the salivary glands;bnm, ventral nervous system;sp, salivary glands;msl, stomach;im, imaginal discs;tr, tracheæ;fk, fatty tissue;ed, intestine;ga, rudiments of reproductive organs;ew, wider portion of intestine;ao, posterior opening.
Fig. 23, Larva ofPlatygaster(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 ofPlatygaster. The letters indicate the same parts as in the preceding figure. 25, Larva of a third species ofPlatygaster. The letters indicate the same parts as in the preceding figures. 26, Larva ofPlatygasterin the second stage—mo, mouth;slkf, œsophagus;gsae, supra-œsophageal ganglion;lm, muscles;bsm, nervous system;ga,gh, rudiments of the reproductive glands. 27, Larva ofPlatygasterin the third stage—mo, mouth;md, mandibles;gsae, supra-œsophageal ganglion;slk, œsophagus;ag, ducts of the salivary glands;bnm, ventral nervous system;sp, salivary glands;msl, stomach;im, imaginal discs;tr, tracheæ;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 tracheæ have made their appearance, and the mouth is provided with a pair of mandibles. From this point the metamorphoses ofPlatygasterdo not appear to differ materially from those of other parasitic Hymenoptera.
An allied genus,Polynema, has also very curious larvæ. The perfect insect is aquatic in its habits, swimming by means of its wings; flying, if we may say so, under water.15It 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 youngPolynemaimbibes nourishment through its whole surface, and increases rapidly in size. The digestive canal gradually makes its appearance; thecellular 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 ofPolynemaoffer no special peculiarities.
Figs. 28-29Fig. 28, Embryo ofPolynema(after Ganin). 29, Larva ofPolynema—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.
Fig. 28, Embryo ofPolynema(after Ganin). 29, Larva ofPolynema—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 larvæ of insects are generally regarded as being nothing more than immature states—as stages in thedevelopment of the egg into the imago; and this might more especially appear to be the case with those insects in which the larvæ 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 hexapodCampodeatype, 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.
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 “larvæ,” 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 larvæ 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 antennæ, mandibles, maxillæ,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 larvæ of the Tenthredinidæ, 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 ofChloëon, 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 larvæ 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 larvæ 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 larvæ 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 hatchedwithout thoracic appendages; in the Orthoptera, on the contrary, the legs are fully developed before the young animal quits the egg.
Prof. Owen,16indeed, goes so far as to say that the Orthoptera and other Homomorphous insects are, “at one stage of their development, apodal and acephalous larvæ, 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 larvæ of theCarabiandPetalocera.”
I quite believe that this may have been true of such larvæ 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 larvæ which leave the egg with these appendages generally make their appearance before the body-walls have closed, or the internal organshave 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.
Figs. 30-31Fig. 30, Egg ofPhryganea(Mystacides)—A1, mandibular segment;C1toC5, maxillary, labial, and three thoracic segments;D, abdomen (after Zaddach). 31, Egg ofPhryganeasomewhat more advanced—b, mandibles;c, maxillæ;cfs, rudiments of the three pairs of legs.
Fig. 30, Egg ofPhryganea(Mystacides)—A1, mandibular segment;C1toC5, maxillary, labial, and three thoracic segments;D, abdomen (after Zaddach). 31, Egg ofPhryganeasomewhat more advanced—b, mandibles;c, maxillæ;cfs, rudiments of the three pairs of legs.
Fig.30,represents an egg of a May-fly (Phryganea), as represented by Zaddach in his excellent memoir,17just 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 inAphis, the embryology of which has been so well worked out by Huxley,18the 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 partin 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.”
InChrysopa oculata, one of the Hemerobiidæ, Packard has described19and 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,20appear at a still earlier stage.
According to the observations of Kölliker,21it would appear that in the Coleopterous genusDonaciathe segments and appendages appear simultaneously.
Kölliker himself, however, frankly admits that “meæ 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 inHydrophilusthe 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.
Fig. 32
Fig. 32.—Egg ofPholcus opilionides(after Claparède).
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 Claparède,23who says, “C’est à cemoment qu’a lieu la formation desprotozonitesou segments primordiaux du corps de l’embryon. Le rudiment ventral s'épaissit suivant six zônes disposées transversalement entre le capuchon anal et le capuchon céphalique.”
Fig. 33
Fig. 33.—Embryo ofJulus(after Newport).
Among Centipedes the development ofJulushas been described by Newport.24The 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 antennæ, but the legs are still only represented by very slight papilliform processeson 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.
Fig. 34Fig. 34.—Colony ofBougainvillea fruticosa, natural size, to the underside of a piece of floating timber (after Allman).
Fig. 34.—Colony ofBougainvillea 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 commonHydraof our ponds, and known technically as the “trophosome,” which develops into the well-known Medusæ or jelly-fishes. The group, however, for which Prof. Allman has proposed the term Monopsea,25and of which the genusÆginamay 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.34represents, after Allman, a colony ofBougainvillea fruticosaof 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 withMedusæ 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 transparentectothèque(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.”
Fig. 35Fig. 35.—Portion of colony ofBougainvillea fruticosa, more magnified.
Fig. 35.—Portion of colony ofBougainvillea fruticosa, more magnified.
Fig. 36
Fig. 36.—The Medusa form of the same species.
Fig.36represents the Medusa form of this species, and the development thus described may be regarded as typical of the Hydroida; yet, as already mentioned, the Æginidæ do not present us with any stage corresponding to the fixed condition of Bougainvillea, but, on the contrary, are developed into Medusæ direct from the egg.
On the other hand, there are groups in whichthe Medusiform stage becomes less and less important.
Figs. 37-38Fig. 37, Larva of Prawn, Nauplius stage (after F. Müller). 38, Larva of Prawn, more advanced, Zoëa stage.
Fig. 37, Larva of Prawn, Nauplius stage (after F. Müller). 38, Larva of Prawn, more advanced, Zoëa stage.
The great majority of the higher Crustacea go through well-marked metamorphoses. Figs.37 and 38represent 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 larvæ occur in various other groups of Crustacea. They were at first regarded as matureforms, and O. F. Müller gave them the name of Nauplius. So also, the second or Zoëa form (Fig.38) was at first supposed to be a mature animal, until its true nature was discovered by Vaughan Thompson.
The Zoëa 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. Branchiæ 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 Zoëa into the perfect animal has been well described by Mr. Spence Bate27in the case of the common crab (Carcinus mænas).
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 Zoëa type; but this is quite an exceptional case. InMysis, says F. Müller,28“there is still a trace of the Nauplius stage; beingtransferred back to a period when it had not to provide for itself, the Nauplius has become degraded into a mere skin; inLigiathis larva-skin has lost the traces of limbs, and inPhilosciait 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.”29Müller 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. 39Fig. 39.—Larva ofEchino-cidaris, seen from above ✕ 6/10 (after Müller).
Fig. 39.—Larva ofEchino-cidaris, seen from above ✕ 6/10 (after Müller).
Fig.39represents the larva of a sea-egg (Echino-cidaris) after Müller.31The body is transparent, shaped somewhat like a double easel, but with two long horns in front, which, as well as the posteriorprocesses, are supported by calcareous rods. This larva swims by means of minute vibratile hairs, or ciliæ. 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
Fig. 40Fig. 40, Larva ofEchinus, ✕ 100.A, front arm;F, arms of the mouth process;B, posterior side arm;E1, accessory arm of the mouth process;a, mouth;a´, œsophagus;b, stomach;b´, intestine;o, posterior orifice;d, ciliated bands;f, ciliated epaulets;c, disc of futureEchinus(after Müller).
Fig. 40, Larva ofEchinus, ✕ 100.A, front arm;F, arms of the mouth process;B, posterior side arm;E1, accessory arm of the mouth process;a, mouth;a´, œsophagus;b, stomach;b´, intestine;o, posterior orifice;d, ciliated bands;f, ciliated epaulets;c, disc of futureEchinus(after Müller).
Fig.40represents 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:ais the mouth,a´the œsophagus,bthe stomach,b´the intestine,fthe ciliated lobes or epaulets,cthe young sea-egg.
Fig. 41Fig. 41.—Comatula rosacea(after Forbes).
Fig. 41.—Comatula rosacea(after Forbes).
The development of the beautifulComatula rosacea(Fig.41) has been described in the “Philosophical Transactions,” by Prof. Wyville Thomson and Dr. Carpenter.33The larva quits the egg, as shown in Fig.42, in the form of an oval body about 1/30 inchin length, something like a barrel, surrounded by four bands orops of long vibratile hairs or ciliæ. 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.
Figs. 42-44Fig. 42, Larva ofComatula rosacea(after Thomson). 43, Larva ofComatula rosacea, more advanced. 44, Larva ofComatula rosacea, in the Pentacrinus state.
Fig. 42, Larva ofComatula rosacea(after Thomson). 43, Larva ofComatula rosacea, more advanced. 44, Larva ofComatula 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 ciliæ, 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 ofComatulawas 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 ofComatula. 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 freeComatula.
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 namedBipinnaria asterigera(Figs.45-47), and which he then supposed to be allied to the ciliograde Medusæ. 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 46represent the front and side viewof a Bipinnaria found by Müller34near Marseilles.ais the mouth,bthe œsophagus,cthe stomach,c´ the intestine. Fig.47represents a somewhat older specimen, in which the Starfish (k) is already beginning to make its appearance.
Figs. 45-47Fig. 45, Larva of Starfish (Bipinnaria), ✕ 100 (after Müller). 46, Larva of Starfish (Bipinnaria), ✕ 100, seen from the side—a, mouth;b, œsophagus;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.
Fig. 45, Larva of Starfish (Bipinnaria), ✕ 100 (after Müller). 46, Larva of Starfish (Bipinnaria), ✕ 100, seen from the side—a, mouth;b, œsophagus;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 genusAsteracanthion—in which development may be said to be direct—the organs and appendages special to the Pseudembryo 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 instanceOphiopholis bellis, “is developed very much after the method ofAsteracanthion Mülleri, 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 inComatula, 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,37and among the insects themselves, in Flies;38and 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.
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;”39this, 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, theAphides—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 coloured when young. The young of the lion and the puma are often striped, and the fœtal 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 donot come within the popular ken. “La larve,” says, Quatrefages, “n’est qu’un embryon à vie indépendante.”40Those 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 Müller 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 Müller 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 thetype 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 samespecies 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 larvæ, in other cases, as we have seen, among some the parasitic Hymenoptera, insects agreeing closely with one another, are produced from larvæ 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 Medusæ.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, sodifferent 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 inChloëon, 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 larvæ 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 aregradual, 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 pupæ 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 pupæ of certain Diptera swim about with much activity. Among the Neuroptera, certain families have pupæ 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 withspecies 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 viewCampodeaand the Collembola (Podura, &c.) are peculiarly interesting. There are in insects three principal types of mouth:—