v. I have not much to say with regard to thesexof pupæ. The male is probably to be distinguished from the female by being smaller; but in the first great division of pupæ, those which resemble the larvæ, and are locomotive, the female in numerous cases may be known by the Ovipositor, or instrument for depositing her eggs in their proper station: and the male also has his anal instruments. Sometimes in this state the animal is so matured, as to be capable of continuing its kind. I have found the pupæ both of aGryllusL. and of aCimexL.in coitu.
vi. Though the pupæ of the second great division are usually not locomotive, yet I must not omit some notice of theirmotions. As the legs of insects in this state are folded within a common or partial integument, of course none of the pupæ now under consideration, with the exception of those of theTrichopteraorder, can walk:coarctateones are even incapable of the slightest motion, and exhibit no symptom whatever of animation. Some of those that are termedincomplete, however, and most chrysalises, have the power of communicating to their bodies a slight movement, extending more or less in different species, which is effected by the abdominal segments solely. The latter, during the first twelve hours of being pupæ, when their skin is soft, frequently turnthemselves, that the side on which they lie may not be flattened; afterwards by far the majority merely wriggle or twist their abdomen when touched, or in any way incommoded or disturbed. We learn from De Geer, that the pupa of the ghost-moth (Hepialus Humuli), the cocoon of which is more than twice the length of the chrysalis, moves in it from one end to the other[645]. Bonnet observed one of a moth (perhapsLasiocampa Quercus), which alternately fixed itself at the top and bottom of its spacious and obliquely-fixed cocoon; descending slowly, but ascending as quickly, and almost in the same manner, as a chimney-sweeper in a chimney[646]. The pupa of the weevil of the water-hemlock (Lixus paraplecticus) will move from one end of the interior of a branch to another by means of itsadminicula, aided by the motion of its abdominal segments[647]. But the most locomotive of pupæ of the second division are those of gnats, and many Tipulidans, which pass this state in the water. These will move from the bottom to the surface, and back again, with great facility and velocity. I have before mentioned several other motions of pupæ[648], which I shall not repeat here, by which they extricate themselves from their several places of intermediate repose, before they leave the puparium: if the imago were to be disclosed in the interior of a tree, or in the earth, its wings would be materially injured in forcing its way out. The object of several of the above motions may be to alarm insects that might attack these defenceless beings. The twirling motion in particular, formerly noticed[649], in some species, bycausing a rustling against the sides of the cocoon, makes a considerable noise—so singular in that of a red underwing-moth (Noctua pacta), that Rösel tells us, (who by the by was more timid than becomes a philosopher,) that the first time he heard it, he had nearly thrown away the box that contained it, in his fright[650].
vii. We are next to considerThe extrication of the perfect insect from the puparium, or pupa-case, and from the cocoon. The period when the pupa has attained maturity, and the inclosed insect is ready to burst the walls of its prison, may be often ascertained. Just at this time the colour frequently undergoes an alteration, the golden or silver tint of the gilded chrysalises vanishes; and those which are transparent, usually permit the form and colours of the insect within and the motions of their limbs to be distinctly seen through them. In theLibellulinathe eyes become more brilliant[651]. The mature pupæ of the moth lately mentioned (Eriogaster lanestris) have a particular swell of the abdominal segments, not apparent in those that are to continue till another season, or longer[652]. Those of the case-worms (Trichoptera) push off the grates from the cases which they have hitherto inhabited, and swim about[653]. Other signs and motions doubtless predict the approach of this great change in other species, which have not been recorded.
The mode in which insects make their way out of thepupariumdiffers in different orders. Inobtectedpupæ, the struggles of the included butterfly or moth first effect a longitudinal slit down the middle of the thorax, wherethere is usually a suture for the purpose. The slit rapidly extends along the head, and down the parts which compose the breast, and the insect gradually withdraws itself from its case. It is not, however, from the outer skin merely that it has to disengage itself, but also from a series of inner membranous cases, which separately inclose the antennæ, proboscis, feet, &c., as a glove does the fingers; and similar cases inclose the parts of the perfect insect in pupæ of all the other orders. This is sometimes a work of difficulty, but ordinarily it is effected with ease.
Incompleteandsemicompletepupæ undergo nearly the same process, save that in them the body is not swathed up in a common case; and therefore they have only to liberate themselves from the partial cases that envelop the several parts of their body.
Incoarctatepupæ, as those ofMuscidæ,Syrphidæ,Œstridæ, &c., the process is different. Their outer-case is ordinarily more rigid and destitute of the sutures, which in the former tribes so easily yield to a slight effort. Yet in these, at the anterior end under which the head of the fly lies, and from which it always issues, there is commonly a sort of lid, joined by a very indistinct suture to the rest, which can be pushed off, leaving a sufficient opening for the egress of the insect. In the pupæ of many of this tribe this lid is composed of two semicircular pieces, which can be separately removed. Many species seem to be able to force off the lid of their puparium, by merely pushing against it with their heads: but the common flesh-fly and many otherMuscidæ, which are perhaps too feeble to effect this, or whose puparia are stronger than ordinary, are furnished with a very remarkableapparatus for this express and apparently sole purpose. They are gifted with the power of introducingairunder the middle part of the head, to which the antennæ are fixed, and of inflating that part into a sort of membranous vesicle as big as the head itself; by the action of which against the end of the pupa-case, the lid is soon forced off. So powerful is this singular lever, that it is even sufficient to rupture the fibrous galls in which the pupæ of the gay-wingedTephritis Cardui[654]are inclosed. That it is designed by Creative Wisdom to answer this sole purpose seems proved, from its disappearing soon after the disclosure of the fly, whose head shortly becomes all alike hard. Reaumur suspects that it may also be intended to promote the circulation of the insect's fluids; but to me his reasons appear not conclusive[655]. In one instance a mode still more unexpected obtains. The illustrious naturalist just named found that the fly which proceeded from one of the rat-tailed grubs (ElophilusLatr.) had actually the power of completely reversing its situation in its narrow case; and that it then employed itstailin pushing off the lid, which other species remove by means of theirheads[656].
The extrication of insects whose pupæ are above ground, like those of butterflies, many beetles, flies, &c., is comparatively a simple operation. But what, you will ask, becomes of those species whose pupæ are concealed deep in the earth, or in the heart of the trees on which their larvæ have fed? Of this you shall be informed.—Coleopterousinsects disclosed from pupæ thus circumstanced,wait until their organs have acquired strength, and their elytra are sufficiently hardened to protect their filmy wings from damage in forcing their way through the earth or wood which covers them. ThusOryctes nasicornis, a rhinoceros beetle common on the Continent, is a fullmonthbefore it reaches the surface of the earth, after quitting its puparium. But it is evident that no delay would enablelepidopterousordipterousinsects, which are without elytra, to make their way out of such situations, without irreparable injury to their delicate wings. Many of these, therefore, while still within the hard case of the pupa, have the precaution, a few days previously to their exclusion, to force themselves up to the surface of the earth, or, when they reside in the interior of trees, to the entrance of their hole. This is effected by a successive wriggling of the abdominal segments, which in several species, of theColeoptera,Lepidoptera, andDipteraorders, for this purpose, as has been more than once observed[657], are furnished with sharp points (adminicula), admitting a progressive, but not a retrograde motion. The puparia of the great goat-moth (Cossus ligniperda) may be often seen projecting from orifices in willow-trees; and those of the common crane-fly (Tipula oleracea) from the surface of the earth, to which they have thus made their way from a depth of several inches.
In all the preceding instances the exclusion of the perfect insect is complete, as soon as it has withdrawn itself from the puparium. But to a very large number, even after this is effected, the arduous task still remains ofpiercing the cocoons of leaves, of thick silk, of tough gum, or even of wood, in which the pupæ are incased. We can readily conceive how the strong jaws ofcoleopterousandhymenopterousspecies may be employed to release them from their confinement. But what instruments can be used for this purpose bymothsin a state of great debility, whose mouth has nothing like jaws—merely a soft membranous proboscis? How shall the silkworm-moth (B. Mori) force its way through the close texture of a silken ball, through which the finger could not be easily pushed? Or the puss-moth (Cerura Vinula) pierce the walls of its house of glue and wood, which scarcely yield to the knife? You will not doubt that these difficulties have been foreseen byInfinite Wisdom, and provided against byInfinite Power. The egress of moths from their cocoons is secured in two ways;—either by some peculiarity in the first construction of the cocoon by the caterpillar, or by some process which the pupa or perfect insect is instructed to perform. As examples of each, several curious instances may be cited.
The larva of the moth which about 1760 made such havoc in the province of Angoumois in France, becomes a pupa in the interior of the grain of wheat which it has excavated; but the opening by which it first entered is not bigger than a pin's point, and is quite insufficient for the egress of the moth. How, then, is the latter to force its way through the tough skin which surrounds it? The larva, previously to assuming the pupa state, gnaws out a little circular piece at that end of the grain where the head of the future moth would lie, taking care not to detach it entirely. At this little door, which is sufficient to protect it from intruders, the moth has but to push, when itfalls down, and leaves a free passage for its exit. A contrivance almost similar is adopted by a caterpillar which feeds in the interior of the heads of a species of teazel (DipsacusL.), for a minute and interesting history of which we are indebted to Bonnet. This caterpillar previously to its metamorphosis actually cuts a circular opening in the head, sufficiently large for the egress of the future moth; but to secure this sally-port during its long sleep, it artfully closes it with fibres of the teazel, closely but not strongly glued together[658]. Another small caterpillar described by the same author, resides in the leaf of an ash curiously rolled up into a cone, and then assumes the pupa, which is inclosed in a silken cocoon, ingeniously suspended by two threads like a hammock in the middle of its habitation, and of so slight a texture that it presents no obstacle to the extrication of the moth. It is the closely-joined sides of its leafy dwelling that form a barrier, which, were it not for the precaution of the larva, would be impenetrable to so small and weak an animal. The little provident creature, before its change to a pupa, gnaws in the leaf a round opening, taking care not to cut through the exterior epidermis. This door is to serve the moth for its exit, like that formed by the wheat-caterpillar. But in proportion to its bulk its verdant apartment is of considerable size. How then shall the moth know the exact place where its outlet has been traced? How, without a clue, shall it discover in its dark abode the precise circle which requires only a push to throw it down? Even this is foreseen and provided against. Out of twenty positions in which its hammockmight have been slung, the caterpillar has been directed so to place it, that the silken cord that suspends the head is fastened close to the side of the door which it has previously constructed; and the moth, guided by thisfilum ariadneum, at once makes its way out of an apartment which, but for this contrivance, might have been to it a labyrinth as inextricable as that of Minos[659].
The mode in which other caterpillars provide for their extrication, when become moths, from their silken cocoons, is not less ingenious. Those ofEriogaster lanestris(of which I have lately said so much,) and others, form oblong cocoons, which, viewed externally, you would at the first glance assert were of one solid piece: but on examining them more narrowly, you perceive one end of them to be a distinct lid, of a size large enough to permit the moth to issue out; and that it is kept in its place by a few slight threads, easily broken by pressure from within[660]. A few pages back[661]I mentioned a cocoon formed by the larva ofTortrix prasinana, of the shape of a boat reversed, composed of two inclined walls fastened together at the top and ends. In constructing this cocoon, it firmly glues to each other the top and one end, so as to form an impermeable suture; but the other end, at which the moth is to issue, though externally it seems as strong as the rest, is merely drawn close by a slender thread or two fastened on the inside, and easily broken from within. And, what is particularly singular in the construction of this ingenious habitation, the sides forming the end last mentioned, though originally requiring force to drawthem into their required position, become so elastic as to close again when the moth has passed between them and made her escape; the cocoon preserving its usual shape, even when deprived of its inhabitant[662]. A similar cocoon is constructed by another leaf-rolling caterpillar, that ofTortrix chlorana[663]. Many similar proofs of contrivance in the construction of silken cocoons might be adduced, but I shall confine myself to one more only—I mean that furnished by the flask-shaped brown one ofSaturnia Pavonia, and some other moths. If you examine one of these cocoons, which are common enough in some places on the pear-tree or the willow, you will perceive that it is generally of a solid tissue of layers of silk almost of the texture of parchment; but at the narrow end, or that which may be compared to the neck of the flask, that it is composed of a series of loosely-attached longitudinal threads, converging, like so many bristles, to a blunt point, in the middle of which is a circular opening[664]. It is through this opening that the moth escapes. The silk of its cocoon is of so strong a texture and so closely gummed, that had both ends been similarly closed, its egress would have been impracticable; it finds, however, no difficulty in forcing its way through the aperture of a sort of reversed funnel, formed of converging threads that readily yield to pressure from within. But an objection will here probably strike you. You will ask, Is not this facility of egress purchased at too dear a rate? Must not a chrysalis in an open cocoon be exposed to the attacks of those ichneumons of which you have said so much, and of numerous other enemies, which will findadmittance through this vaunted door? Our caterpillar would seem to have foreseen your dilemma; at least, under heavenly guidance, she has guarded against the danger as effectually as if she had. If you cut open the cocoon longitudinally, you will see that within the exterior funnel-shaped end, at some distance she has framed a second funnel, composed of a similar circular series of stiff threads, which, proceeding from the sides of the cocoon, converge also to a point, and form a sort of cone exactly like the closed peristome of a moss; or, to use a more humble though not less apt illustration, like the wires of certain mousetraps[665]. In this dome not the slightest opening is left, and from its arched structure it is impenetrable to the most violent efforts of any marauders from without; whilst it yields to the slightest pressure from within, and allows the egress of the moth with the utmost facility. When she has passed through it, the elastic threads resume their former position, and the empty cocoon presents just the same appearance as one still inhabited. Rösel relates with amusing naïvété how this circumstance puzzled him the first time he witnessed it: he could scarcely help thinking that there was something supernatural in the appearance of one of these fine moths in a box in which he had put a cocoon of this kind, but in which he could not discover the slightest appearance of any insect having escaped from it, until he slit it longitudinally[666]. But from an observation of Meinecken, it appears that these converging threads servea double purpose; being necessary to compress the abdomen of the moth as it emerges from the cocoon, which forces the fluids to enter the nervures of the wings, and give them their proper expansion. For he found, that when the pupa is taken out of the cocoon, the moth is disclosed at the proper time, but remains always crippled in its wings; which never expand properly, unless the abdomen be compressed with the finger and thumb, so as to imitate the natural operation[667].
I am next to give you some account of thesecondmode in which the release of the perfect insect from its cocoon is effected—that, namely, wherein its own exertions chiefly accomplish the work. I shall from a large number select only a few instances. The texture of the cocoon of the silkworm-moth is uniform in every part, and the layers of silk are equally thick at both ends. The moth makes its way out by cutting or breaking these threads at the end opposite to its head: an operation which, as it destroys the continuity of the silk, those who breed these insects are particularly careful to guard against, by exposing the cocoon to heat sufficient to destroy the included pupa. The question is—What instruments does the moth employ to effect this? And this we are not able to answer satisfactorily. Malpighi asserts that the animal first wets the silk with a liquid calculated to dissolve the gum that connects the threads, and then employs its lengthened head to push them aside and make an opening[668]. But, as Reaumur has observed, besides that so obtuse a part as the head of a moth is but ill fitted to act as a wedge, we findthe threads not merely pushed to each side, but actually cut asunder. He therefore infers that the eyes, which are the only hard organs of the head, are the instruments by which the threads are divided—their numerous minute facets serving the purpose of a fine file[669]. It should be observed, however, that Mr. Swayne confirms Malpighi's assertion, that the silkworm does not cut, but merely pushes aside, the threads of its cocoon; and he informs us that he has proved the fact, by unwinding a pierced cocoon, the thread of which was entire[670]. Yet Reaumur's correctness cannot be suspected: and he affirms, that from observation there can scarcely be a doubt that most of the threads are broken[671]; which is further confirmed in an account of the breeding of silk-worms published in theAmerican Philosophical Transactions: in which it is expressly stated, that cocoons out of which the fly has escaped, cannot be wound[672]. Analogy, it must be confessed, is against Reaumur's opinion; since other kinds of silkworms make their escape by means of afluid. Thus we are informed by Dr. Roxburgh, thatAttacus Paphia, when prepared to assume the imago, discharges from its mouth a large quantity of liquid, with which the upper end of the case is so perfectly softened, as to enable the moth to work its way out in a very short space of time,—an operation which, he says, is always performed in the night[673]. Perhaps the two opinions may be reconciled, by supposing the silkworm first to moisten and then break the threads of its cocoon. In those that are of a slighter texture, a mere push against themoistened end is probably sufficient: and hence we find in so many newly disclosed moths the hair in that part wet, and closely pressed down[674]. If it be apparently difficult for the silkworm-moth to effect an opening in its cocoon, how much harder must seem the task of the puss-moth (Cerura Vinula) to pierce the solid walls of its wood-thickened case? Here the eyes are clearly incompetent; nor could any ordinary fluid assist their operation, for the gum which unites the ligneous particles is indissoluble inaqueousmenstrua. You begin to tremble for the fate of the moth incarcerated in such an impervious dungeon—but without cause: what anaqueoussolvent cannot effect, anacidis competent to: and with a bag of such acid our moth is furnished. The contents of this she pours out as soon as she has forced her head through the skin of the chrysalis, and upon the opposite end of the cocoon. The acid instantly acts upon the gum, loosens the cohesion of the grains of wood, and a very gentle effort suffices to push down what was a minute ago so strong a barrier. How admirable and effectual a provision! But there is yet another marvel connected with it. Ask a chemist, of what materials a vessel ought to be to contain so potent an acid: he will reply,—of glass. Yet our moth has no glass recipient: her bottle is a membranous bag; but of so wonderful a fabric as not to be acted upon by a menstruum which a gum, apparently of a resinous nature, is unable to resist! This fact can only be explained by the analogous insensibility of the stomach to the gastric juice, which in some animals can dissolve bone,—and it is equally worthy ofadmiration. In both cases, the vitality of the membranous or fleshy receptacle secures it from the action of the included fluid; buthow—who shall explain?
Ordinarily it is the moth that breaks the cocoon; but in the goat-moth and manyTortricesit is the pupa itself that performs the work, either wholly or partially. The pupa of the former is for this purpose furnished with sharp points upon the head, capable of effecting this object[675]. The locust-moth, another species ofCossus(C. RobiniæPeck), whose history has been admirably detailed by Professor Peck, has a different process. "In the silk-moth," says he, "and all others which I have had opportunity to observe, the chrysalis burstsinthe cocoon, and the fluid which surrounded the new insect in it escaping at the same time, so weakens or dissolves the fibre and texture of the silk, that the moth is able to extricate itself, leaving the chrysalis behind it; but this is not the manner in the locust-moth. After remaining till all its parts are fully grown and it is ready to quit its prison, a certain quantity of exercise is necessary, to break the ligaments which attach the moth to the shell of the chrysalis, and to loosen the folds of the abdomen. In taking this exercise, it can only move the abdomen in various directions: as one side of the rings is moved forward, the hooks in the serrated lines above mentioned (theadminicula) take hold of the silk, and prevent their sliding back; the next flexure brings forward the opposite side of the rings, which are prevented by the points on that side from slipping back in the same manner, and the chrysalis is forced out of the slightly woven extremityof the cocoon, and through the silk-lined cavity, till it is protruded for about one-third of its length out of the opening in the bark, and into the air[676]."
An exception to the general rule—that the rupturing of the cocoon is the business of the inclosed insect itself—is met with amongst ants; the workers of which not only feed the young, but actually make an aperture in their cocoons, cutting the threads with their mandibles with admirable dexterity and patience, one by one, at the time they are ready to emerge, the precise period for which these indefatigable nurses are well aware of, that they may meet with no obstacle. Without this aid, the young ant would be unable to force its way through the strong and dense coating of silk that infolds it[677]. And a proceeding somewhat akin to this was observed by the Hon. Captain Percy, R.N., who himself related it to me. Being fond of the study of insects, he was in the habit of attending to their motions; and in the beginning of September 1821 noticed those of a number of femaleTipulæ, probablyT. oleraceaL., busily engaged in depositing their eggs amongst the roots of grass. While observing these proceedings, he at the same time saw one quitting its pupa-case, which had already by its own efforts got its head, thorax, and anterior legs out of it. It was then joined by twomaleflies; which, with their anal forceps and posterior legs taking hold of the pupa-case, appeared with their mouths and anterior legs to push the little prisoner upwards, moving her backwards and forwards; and as they kept raising her, shifting their hold of theskin till she was entirely extricated, when they left her to recover her strength by herself. Probably the extreme length of the two pair of hind-legs of these animals may render such assistance necessary for their extrication.
There remains yet to be explained under this head the manner in which the perfect insect is excluded from certain aquatic pupæ; such as those ofPhryganeæ, gnats, and one of thoseTipulidæthat resemble gnats. These pupæ (perhaps that they may be safe from the attack of birds) are destined to remain during the greater part of their existence in this state at the bottom of the water. But it is obvious that if the perfect insects were there to be disclosed, their wings would be wetted, and they would be drowned. It is the provision by which this result is obviated that now calls for your attention.
You have already been told that the larvæ ofPhryganeæinclose themselves in cases of different materials, open at each end[678]. You have also learned, that in becoming pupæ, they secure each end of their cases with a grating of silk[679]. When that change has occurred, they remain motionless at the bottom of the water. Now how are these pupæ, encased in tubes of a greater specific gravity than the surrounding fluid, to make their way to the surface when the time has arrived for their becoming denizens of the air? This they accomplish in the following manner:—The pupa is furnished with two strong exterior moveable mandibuliform processes, and has the power of moving its four anterior legs and antennæ while in the pupa-case. With these temporaryjawsit makes an opening in one of the silken doors of its case, forces itsway out at that end, and then by moving its legs, the cases of which in some species are ciliated for this very purpose, swims to the surface, where its skin splits, and discloses the included insect. That these jaws are given for the express and exclusive purpose of being thus applied, seems undeniable. The pupa eats nothing—they are therefore in every other point of view superfluous. They are given to it alone of all other similar pupæ, because unnecessary to all others; and they are cast off along with the rest of the puparium, the perfect insect having no vestige of jaws[680].
Thegnathas to undergo its change on the surface of the water—How is it to accomplish this without being wetted? In the pupa state they usually remain suspended with the posterior end of the body turned downwards: but when the period for its change is arrived, it stretches it out upon the surface, above which its thorax is elevated. Scarcely has it been a moment in this position, than, swelling out the interior and anterior parts of the thorax, it causes it to split between the two respiratory horns. Through this opening the anterior part of the gnat then emerges. As soon as the head and trunk are disengaged, it proceeds with its labour, and gets out more and more; elevating itself so as to appear in the puparium like a mast in a boat. As it proceeds, the mast is more and more elevated and lengthened, till it becomes nearly perpendicular—just as the mast of a boat is gradually raised from a nearly horizontal to a vertical position: at this period a very small portion of the abdomen remains in the puparium. Neither its legs nor wings are of any usein maintaining it in this position. The latter are too soft, and, as it were, folded; and the former are stretched out along the abdomen—the segments of this last part are the only agents. The observer who sees how the little boat gradually sinks, and how its margin approaches the water, forgets the mischievous insect it contains, which at another time he would crush without remorse, and becomes interested for its fate; especially should wind agitate the water. A very little is sufficient to drive about rapidly the little voyager, since it catches the wind in some degree as a sail. If it should be upset, it would be all over with it;—and numbers do thus perish. The gnat, after having fixed itself thus perpendicularly, draws first its two anterior legs out of their case, and moves them forward, and next the two intermediate ones; then inclining itself towards the water, it rests its legs upon it, for water is to them a soil sufficiently firm and solid to support them, although surcharged with the weight of the insect's body. As soon as it is thus upon the water, it is in safety; its wings unfold themselves and are dried, and it flies away. All this is the work of an instant[681].
The pupæ ofChironomus plumosusproceed from those red worm-like larvæ so common throughout the summer in tubs of rain-water, &c., described by Reaumur[682]. They are not inclosed in cases, but are of a greater specific gravity than the water at the bottom of which they reside, until within a few hours of the exclusion of the fly. They have the power of swimming, however; and by moving the tail alternately backwards and forwards, can slowly raise themselves to the top of the water. Buthere occurs a difficulty. For the extrication of the imago it is necessary that they should remain quietly suspended at the surface; and moreover that the thorax, in which the opening for its exit is to be made, should be at least level with it: and this is precisely what takes place. If you watch one of these pupæ when it ascends from the bottom, you will see that as soon as it has reached the top it remains suspended there motionless; and that its thorax is the highest part of the body, and level with the surface. Now the question is, in what way this is accomplished? How can a pupa of greater specific gravity than water, remain suspended without motion at its surface? and how can its thorax, which is at its heaviest end, be kept uppermost?—By a most singular and beautiful contrivance, which I shall explain; the more particularly because it has escaped Reaumur, and, as far as I know, all other entomological observers. The middle of the back of the thorax has the property of repelling water—apparently from being covered with some oily secretion. Hence, as soon as the pupa has once forced this part of its body above the surface, the water is seen to retreat from it on all sides, leaving an oval space in the disk, which is quite dry. Now though the specific gravity of the pupa is greater than that of water, it is but so very slightly greater, that the mere attraction of the air to the dry part of the thorax, when once exposed to it, is sufficient to retain it at the surface; just as a small dry needle swims under similar circumstances. That this is a true solution of the phænomenon, I am convinced by the result of several experiments. If, when the pupa is suspended at the surface, a drop of water be let fall upon the dry portion of the thorax, it instantly sinks to thebottom,—the thorax, which belongs to the heaviest half, being the lowest; and if the pupa be again brought to the surface, so that the fluid is repelled from its disk, it remains suspended there without effort, as before. Just previously to the exclusion of the fly, the dry part of the thorax is seen to split in the middle. The air enters, and forms a brilliant stratum resembling quicksilver, between the body of the insect and its puparium; and the former pushing forth its head and forelegs, like the gnat, rests the latter upon the water, and in a few seconds extricates itself wholly from its envelope.
Before I close this letter, I must state a fact connected with the subject of it that deserves to be recorded. It is a general rule, thatonepupa-case incloses onlyoneinsect; but Kleesius, a German entomologist, asserts that he had oncetwospecimens ofGastropacha quercifoliaproduced fromonepupa; which was large, being full two inches long, and one thick.
When the insect has quitted the exuviæ of the pupa, it has attained the last stage of its existence. It is now termed anImago, or perfect insect; and is capable of propagation.
Just after its exclusion, it is weak, soft, and languid: all its parts are covered with moisture; and, if a winged insect, its wings have so little the appearance, either in shape, size, or colour, which they are about to assume, that it might be taken for a mutilated abortion, rather than an animal in the most vigorous stage of life. If it be a beetle, its elytra, instead of covering the back of the abdomen, are folded over the breast: their substance is soft and leathery, and their white colour exhibits no traces of the several tints which are to adorn them. If the insect be a butterfly or a moth, the wings, instead of being of their subsequent amplitude, and variegated and painted with a variety of hues and markings, are in large species scarcely bigger than the little finger nail, falling over the sides of the trunk, and of a dull muddy colour, in which no distinct characters can be traced.If the excluded insect be a bee or a fly, its whole skin is white and looks fleshy, and quite unlike the coloured hairy crust which it will turn to in an hour or two; and the wings, instead of being a thin, transparent, expanded film, are contracted into a thick, opaque, wrinkled mass.
These symptoms of debility and imperfection, however, in most cases speedily vanish. The insect, fixing itself on the spoils of the pupa, or some other convenient neighbouring support, first stretches out one organ, and then another: the moisture of its skin evaporates, the texture becomes firm, the colours come forth in all their beauty; the hairs and scales assume their natural position; and the wings expanding, extend often to five or six times their former size—exhibiting, as if by magic, either the thin transparent membranes of the bee or fly, or the painted and scaly films of the butterfly or moth, or the coloured shells of the beetle. The proceedings here described I witnessed very recently with regard to a very interesting and beautiful butterfly, the only one of its description that Britain has yet been ascertained to produce—I meanPapilio Machaon. The pupa of this being brought to me by a friend early in May this year (1822), on the sixteenth of that month I had the pleasure to see it leave its puparium. With great care I placed it upon my arm, where it kept pacing about for the space of more than an hour; when all its parts appearing consolidated and developed, and the animal perfect in beauty, I secured it, though not without great reluctance, for my cabinet—it being the only living specimen of this fine fly I had ever seen. To observe how gradual, and yet how rapid, was the development of the parts and organs, andparticularly of the wings, and the perfect coming forth of the colours and spots, as the sun gave vigour to it, was a most interesting spectacle. At first it was unable to elevate or even move its wings; but in proportion as the aërial or other fluid was forced by the motions of its trunk into their nervures, their numerous corrugations and folds gradually yielded to the action, till they had gained their greatest extent, and the film between all the nervures became tense. The ocelli, and spots and bars, which appeared at first as but germes or rudiments of what they were to be, grew with the growing wing, and shone forth upon its complete expansion in full magnitude and beauty.
To understand more clearly the cause of this rapid expansion and development of the wings, I have before explained to you that these organs, though often exceedingly thin, are always composed of two membranes, having most commonly a number of hollow vessels, miscalled nerves, running between them[683]. These tubes, which, after the French Entomologists, I would namenervures, contribute as well to the development of the wings, as to their subsequent tension. In the pupa, and commonly afterwards, the two membranes composing the organs in question do not touch each other's inner surface, as they afterwards do: there is consequently a space between them; and being moist, and corrugated into a vast number of folds like those of a fan, but transverse as well as longitudinal, and so minute as to be imperceptible to the naked eye, the wings appear much thicker than in the end. Now as soon as the insect is disclosed, a fluid entersthe tubes, and being impelled into their minutest ramifications, necessarily expands their folds; for the nervures themselves are folded, and as they gradually extend in length with them, the moist membranes attached to them are also unfolded and extended. In proportion as this takes place, the expanding membranes approach each other, and at last, being dried by the action of the atmosphere, become one. To promote this motion of the fluid, seems the object of the agitations which the animal from time to time gives to its unexpanded wings. That a kind of circulation, or rather an injection of an aqueous fluid into these organs, actually takes place, may be ascertained by a very simple experiment. If you clip the wings of a butterfly during the process of expansion, you will see that the nervures are not only hollow, but that, however dry and empty they may subsequently be found, they at that time actually contain such a fluid[684]. Swammerdam, who appears to have been the first physiologist that paid attention to this subject, was of opinion that an aëriform as well as an aquiform fluid contributes to produce the effect we are considering. He had observed that, if a small portion be cut off from the wing of a bee, a fluid of the latter kind exuded from its vessels in the form of pellucid globules, becoming insensibly drops—which he concluded proved the action of the latter; and he noticed, also, that the wings were furnished with tracheæ, which were at that time distended by the injected air; whence he justly surmised, that the action of theairwas also of great importance to produce the expansion of the wing[685]. And Jurine found that every nervure containsa trachea, which, proceeding from the interior of the trunk in a serpentine direction, follows all the ramification of the nervure, though it does not fill it[686]. Though Reaumur attributes the expansion of the wings chiefly to anaqueousfluid, yet he suspects that theairon some occasions contributed to it[687].
The wings of the other tribes of insects probably differ from theLepidopterain the manner in which they are folded. It should seem from Reaumur's description, that those of some flies, instead of the straight transverse folds of the former, have angular or zigzag folds[688]; which equally shorten the wing. ManyHymenopterahave wings without any nervures except the marginal. We may conjecture that these are more simply folded, so as to render their expansion more easy; but even in these wings there are often tracheæ, which appear as spurious nervures, and help to effect the purpose we are considering.
The operation of expanding their wings, in by far the larger number of insects, takes place gradually as described above; and, according to their size, is ended in five, ten, or fifteen minutes; in some butterflies half anhour, in some even an hour. A few species, such asSphinx ŒnotheræF., require several hours, or even a day, for this operation; and, from the distance to which they creep before it has taken place, a considerable degree of motion seems requisite for causing the necessary impulse of the expanding fluids[689]. In a few genera, however, as the gnat, the gnat-like Tipulidæ, and the Ephemeræ, this process is so rapid and instantaneous, that the wings are scarcely disengaged from the wing-cases before they are fully expanded and fit for flying. These genera quit the pupa at the surface of the water, from which, after resting upon it for a few moments, they take flight: but this would evidently be impracticable, and immersion in the fluid, and consequent death, would result, were not the general rule in their case deviated from.
Some species of the last of these genera,Ephemera, are distinguished by another peculiarity, unparalleled, as far as is known, in the rest of the insect world. After being released from the puparium, and making use of their expanded wings for flight, often to a considerable distance, they have yet to undergo another metamorphosis. They fix themselves by their claws in a vertical position upon some object, and withdraw every part of the body, even the legs and wings, from a thin pellicle which has inclosed them, as a glove does the fingers; and so exactly do the exuviæ, which remain attached to the spot where the Ephemera disrobed itself, retain their former figure, that I have more than once at first sight mistaken them for the perfect insect. You can conceive without difficulty how the body, and even legs, can be withdrawnfrom their cases; but you must be puzzled to conjecture how the wings, which seem as thin, as much expanded, and as rigid as those of a fly, can admit of having any sheath stripped from them; much less how they can be withdrawn, as they are, through a small opening at the base of the sheath. The fact seems to be, that though the outer covering is rigid, the wing inclosed in it, notwithstanding it is sometimes more than twenty-four hours before the change ensues, is kept moist and pliable. In proportion, therefore, as the insect disengages itself from the anterior part of the skin, the interior or real wings become contracted by a number of plaits into a form nearly cylindrical, which readily admits of their being pulled through the opening lately mentioned; and as soon as the insect is released from its envelope, the plaits unfold, and the wing returns to its former shape and dimensions. Thus our little animal, having bid adieu to its shirt and drawers, becomes, but in a very harmless sense, a genuinedescamisadoandsansculotte. It does not seem improbable, that the pellicle we have been speaking of is analogous to that which, in addition to the outer skin, incloses the limbs ofLepidoptera, &c. in the pupa state, but which they cast at the same time with the puparium, and leave adhering to it[690].
The body of newly-disclosed insects commonly appears at first of its full size; but the aphidivorous flies (SyrphusF. &c.), and some others, in about a quarter of an hour after leaving the pupa become at least twice as large as they were at their first appearance: this apparent sudden growth, which is also noticed by Goedart,Reaumur found to depend upon the expansion of the previously compressed segments of the animal by means of the included air[691]. Both in this instance and in that of insects whose wings only require expansion, the size of theimagooften so greatly exceeds that of thepupa, that we can scarcely believe our eyes that it should have been included in so contracted a space. The pupa of one of the beautiful lace-winged flies (Hemerobius Perla) is not so big as a small pea, yet the body of the fly is nearly half an inch long, and covers, when its wings and antennæ are expanded, a surface of an inch square[692].
When the development of the perfect insect is complete, and all its parts and organs have attained the requisite firmness and solidity[693], it immediately begins to exercise them in their intended functions; it walks, runs, or flies in search of food; or of the other sex of its own species, if it be a male, that it may fulfill the great end of its existence in this state—the propagation of its kind. Previously to thus launching into the wide world, or at least immediately afterwards, almost all insects discharge from their intestines some drops of an excrementitious fluid, often transparent, and sometimes red. I have before related to you the alarm that this last circumstance has now and then produced on the minds of the ignorant and superstitious[694]. Whether this excrementis produced indifferently both by males and females I cannot positively assert; but a circumstance related by Jurine affords some ground for a suspicion that it is peculiar to the latter. A specimen of a female ofLasiocampa Rubi, when killed emitted some of this fluid, which dropped upon the floor: this appeared to attract the males to the apartment in which it happened, and to the very spot—from whence it may be conjectured, that the scent of the fluid brought them there, and that the use of it is to bring the sexes together soon after exclusion from the pupa[695].
The colour, sculpture, and other peculiarities which distinguish insects in this state I shall consider at large in another letter, when I treat of their external parts and organs. Under the present head I shall confine myself to pointing out the characters by which thesexesof many species are distinguished from each other; as likewise theduration of their lifein their perfect state; together with the circumstances on which this duration depends.
I.Sexual Distinctions.The first general rule that may be laid down under this section is,—That among insects, contrary to what mostly occurs in vertebrate animals, thesizeof the female is almost constantly larger than that of the male. Even in the larva and pupa states, a practised eye can judge, from their greater size, which individuals will become females. There are, however, some exceptions to this rule. Thus amongst theColeoptera, the maleDynastidæ, remarkable for their horns,as you may see inD. Alocus,Antæus,Actæon, &c., as likewise those ofLucanus, are larger than the unarmed females[696]. In theNeuropterathe femaleLibellulidæare sometimes sensibly smaller, and never larger, than their males[697]. In theHymenopterathe male of the hive-bee, but more particularly that ofAnthidium manicatumand other bees of that genus, is much more robust than the other sex[698]. In theDiptera, the same difference is observable inSyrphus Ribesii, and some other aphidivorous flies, and also inScatophaga stercoraria[699]. And amongst theapteroustribes, we are informed by De Geer that the male ofArgyroneta aquatica, which builds an aërial palace in the bosom of the waters[700], usually exceeds the female in bulk[701]. The reason of this rule seems in some degree connected with the office of the female as a mother, that sufficient space may be allowed for the vast number of eggs she is destined to produce; and it is when impregnation has taken place, and the eggs are ready for extrusion, that the difference is most sensible. In the majority of cases this sexual disproportion is not very considerable, but in some few it is enormous. Reaumur mentions a beetle, of which he intended to give the history, the male of which is so small compared with the female, that a bull not bigger than a sheep, or even a hare, set by the side of the largest cow, would aptly contrast with them. This little beetle, he says, has wingsand elytra, while the giant female has no vestige of either, having the upper surface of its body naked and membranous[702]. The species to which this illustrious Naturalist here alludes, does not appear to have been ascertained. The female of many gall-insects (Cocci) is so large in comparison with the male, that the latter traverses her back as an ample area for a walk[703]. But this is nothing compared with the prodigious difference between the sexes ofTermes fatale, and other species of white ants, whose males are often many thousand times less than the females, when the latter are distended with eggs[704]. Accidental differences in the size of the sexes sometimes arise: as when the female larva has, from any cause, been deprived of its proper supply of food, it will occasionally be less than the male. De Geer has stated a circumstance with respect to theAphidesthat produce galls, that should be mentioned under this head—the first, ormotherfemale, is larger than any of her progeny ever become[705].
The second observation that may be generally applied to the sexes of insects is, that, size excepted, there is a close resemblance between them in other respects. But to this rule the exceptions are very numerous, and so important that it is necessary to specify examples of each under distinct heads.
i. In some species the sexes are either partly or wholly of a differentcolour. Thus, in the orderColeoptera, the elytra of the male ofRhagium meridianumF. are testaceous, and those of the female black.Leptura rubraofLinné, with red elytra, is the female of hisL. testacea, in which they are testaceous.Cantharis dermestoidesof the same author is the other sex of hisMeloe Marci; one of which is chiefly testaceous, and the other black: which seems to have so misled Linné, that he placed them in different genera. One more instance in this order, the female ofCicindela campestris, as was first observed to me by our friend Sheppard, has a black dot on each elytrum, not far from its base near the suture, which the male has not.
Amongst theOrthoptera, the maleLocustæF., as Professor Lichtenstein has informed us[706], have a fenestrated ocellus, which is not to be found in the other sex. I was once attending to the proceedings of a Hemipterous species,Pentatoma oleraceaLatr., which I found in union: the paired insects had white spots, but another individual was standing by them, in which the spots were of a sanguine hue. I mention this by the way only—the spots in the prolific sexes being of the same colour: but might not the red spotted one be a neuter?
The sexes of manyLepidopteralikewise differ in their colour. I must single out a few from a great number of instances. The males ofLycæna ArgusF. have the upper surface of their anterior wings of a dark blue, while in the female it is wholly brown. The wings of the former sex ofHypogymna disparare gray, clouded with brown; but those of the latter are white, with black spots. In the brimstone butterfly (Colias Rhamni), which is one of the first that appear in the spring, the wings of the male are yellow—of the female whitish. In the commonorange-tip (Pieris CardaminesF.), one sex has not the orange tip to the upper wings: and, to name no more, the male ofLycæna dispar, one of our rarest and most beautiful butterflies, has only a single black spot in the disk of its fulgid wings; while in the other sex, the primary pair have nine, and the secondary are black, with a transverse orange fascia near the posterior margin. But the most remarkable difference in this respect observable in the insects of the order in question, takes place in a tribe, of which only one species is certainly known to inhabit Britain—I mean thePapiliones Equitesof Linné: what he has called hisTrojaniandAchiviin some instances have proved only different sexes of the same species. Mr. MacLeay's rich cabinet affords a singular instance confirming this assertion;—a specimen of a Papilio is divided longitudinally, the right hand side being male, and the left hand female. The former belongs toP. Polycaon, a Grecian, the latter toP. Laodocus, a Trojan. An instance of twoGreciansthus united is recorded in theEncyclopédie Méthodique, as exhibited in a specimen preserved in the Museum of Natural History at Paris; which on the right hand side isP. Ulysses, on the leftP. Diomedes[707].
In theNeuroptera, theLibellulidæare remarkable for the differences of colour in the sexes. In the commonLibellula depressa, which you may see hawking over every pool, the abdomen of the male is usually slate-colour, while that of his partner is yellow, but with darker side-spots. Reaumur, however, noticed some males that were of the same colour with the females[708]. Schelverobserved, when he put the skins ofLibellula depressainto water, that the colours common to both sexes were in the substance of the skin, and remained fixed; while those that were peculiar to one could be taken off with a hair-pencil, and coloured the water: which therefore were superficial, and, as it were, laid on[709]. The yellow males, therefore, that Reaumur observed, were probably such as had the superficial blue colour which distinguishes them washed off. InCalepteryx VirgoLeach, the former are of a lovely silky blue, and the latter green. InAgrionsF. nature sports infinitely in the colours of the sexes.
In the orderHymenopterathere are often differences equally great; the sexes of many of the Ichneumons and Saw-flies are of quite different colours. The former tribe Linné has divided into sections, from the white annulus observable in the antennæ of some, and from the colour of their scutellum: but these are often merely sexual characters[710]. The male ofAnthophora retusaLatr., a kind of wild bee, is wholly black, the female wholly gray, and of so very different an aspect that they were long regarded as distinct species; a mistake which has likewise occurred with regard to the sexes ofOsmia cærulescens, another bee, of which the male has a bronzed and the female a violet abdomen[711]. The nose of maleAndrenæLatr. is often yellow, or white, as inA. hæmorrhoidalis—when that of the female is black[712]. Thelabrumalso is often of a different colour in the sexes, as inCeratinaLatr.
In theDiptera,Aptera,Arachnida, &c., I am not aware of any striking difference in the colours of the sexes.
ii. The sexes of insects vary (but more rarely than in colour) in theirsculpturealso, andpubescence. Thus the elytra of the females of many of the larger water-beetles (Dytiscus) are deeply furrowed, while those of the males are quite smooth and level[713]. The thorax of the female in several species ofColymbetesof the same tribe, asC. Hybneriandtransversalis, on each side has several tortuous impressed lines or scratches, like net-work, which are not to be discovered in the male.Hyphydrus gibbusLatr., which differs solely fromH. ovalis(Dytiscus ovalisIllig.) in being thickly covered with minute impressed puncta, is, from the observation of the Rev. R. Sheppard, the other sex of this last, with which he has taken it coupled; and it is by no means improbable thatHydroporus picipes(Dytiscus punctatusMarsh.) andH. lineatus,—between which, as Gyllenhal has justly observed, the same difference only exists,—are in like manner sexual varieties. With respect topubescence, I have not much to say. Another aquatic beetle,Acilius sulcatusLeach, has not only its elytra sulcated, but the furrows of these, and a transverse one of the thorax, are thickly set with hair; while the male is smooth, and quite naked. Particular care seems to have been taken by the Creator, that when all the above inhabitants of the water are paired, the male should be able to fix himself so firmly, by meansof his remarkable anterior tarsi, (which I shall afterwards describe,) and these asperities, &c. in the upper surface of his mate, as not to be displaced by the fluctuations of that element, the reluctance of the coy female, or any other slighter cause.
In a moth called the ghost (Hepialus Humuli), the posterior tibia of the male is densely bearded, but not of the female[714].—SomeHymenoptera, asAmmophilaKirb. andStigmusJurine, have the upper lip of the male clothed with silver pile, while that of the female is not so ornamented. The legs of some bees are distinguished in the sexes by a difference in theirclothing. That observable in those of the hive-bee has been before noticed[715]. InAndrenaof Latreille[716]the posterior tibia of the female is covered externally with a dense brush of hairs, for collecting the pollen; and the posterior legs at their base have a curled lock of hair—which are not to be found in the male[717]. InDasypoda,Melecta,Anthophora,Centris,Epicharis, &c. of the same author, the first joint of the tarsus of the female, and inXylocopaalmost the whole tarsus, is also similarly signalized from that of the other sex. InBombus, as in the hive-bee, the posterior tibiæ of the females and neuters are furnished with a basket of hairs for carrying their pollen paste, which you will in vain look for in the male[718]. The latter, however, in some species of this tribe are distinguished from the former by the longer hairs of their legs, but not in the posterior ones. Thus, inAnthophora retusathe first joints of theintermediate tarsus are bearded internally with a thin fringe of long hairs, and the first externally with a triangular one of short ones at the apex: but what is most remarkable, the last or unguicular joint, which in almost every other bee is naked, is on both sides fringed with long hairs[719]. In that remarkable genusAcanthopusIllig., of which the male only is known, the first and last joint of the intermediate tarsus have a dense external brush of stiff hairs, which probably is also a sexual character[720]. Another sexual kind of clothing is exhibited by the females of those bees that have their labrum or upper-lip inflexed (MegachileLatr.)[721]. Their abdomen is covered underneath with a brush of stiff hairs, involved in which they carry the pollen they collect. In the males of some of this tribe, as ofM. Willughbiella, the first four joints of the anterior tarsus on their inner side have a long dense fringe of incurved hairs[722]: a circumstance also to be found in the same sex ofXylocopa latipes, in which the claw-joint also is bearded[723]. InAndrenaLatr. the last dorsal segment of the abdomen of the same sex is fringed, while that of the male is naked[724]. In the humble-bees (Bombus), the mandibles of the male are bearded with curled hairs, while those of the females and neuters are without them. Some bees, asAndrenaandHalictusLatr., have the anus of the female bearded, and that of the male naked: in someBombycesthe reverse takes place.
iii. With regard to the generalshapeof their body, the male and female usually resemble each other: but there are some exceptions to this rule. The male of the hive-bee is much thicker and more clumsy than either the female or the worker; but inHalictusLatr. the males are nearly cylindrical in shape, and very narrow; while the other sex are oblong or ovate, especially their abdomen: and inAndrenaLatr. the former are much slenderer than the females, and of a more lanceolate shape. But a still more striking difference in this respect between the sexes is exhibited by some species of the genusPtinusF., in which the male is long and slender, and the female short and thick. This, in more than one instance, has occasioned them to be mistaken for distinct insects: thus,P. LichenumandP. similis,P. ovatusandP. testaceus, of Mr. Marsham, are mere sexual varieties. But the most entire abalienation of shape at present known, is that which distinguishes the male from the femaleCoccus; these are so completely dissimilar as scarcely to have any part in common. InBombyx vestitaF., and others of the same family, while the males are of the ordinary conformation of the order, the females are without even the slightest rudiments of wings; they have no antennæ, the legs are extremely short, not longer than those of the caterpillar; and the body is entirely destitute of scales, so that they altogether assume the exact appearance of hexapod larvæ[725]. A conformation nearly similar takes place in the female ofTinea Lichenella; but in this the feet are longer, and the anus is furnished with a long retractile ovipositor[726].