Part 14
"It began," says Réaumur, "by coming almost entirely out, and left only its hinder part within. It moved its head forwards as far as was necessary to enable it to seize a particle of earth. As soon as it had got its load, it re-entered the interior of the cocoon. It deposited the grain of earth, and came out again immediately, as it did at first, to pick up another grain, which it carried likewise into the interior of the cocoon. This operation it continued for more than an hour.... The provision of materials being got together, the caterpillar now devoted his whole attention to working them up. It began by spinning over one part of the edges of the opening. After having put over this a small band of very loose web, the caterpillar's head left the opening, the insect went right back again into its cocoon, and the head returned to the opening loaded with a little grain of earth, which it entangled in the silky threads. It then entangled in them two or three, or a greater number of grains, according to the quantity of threads it had spun. It bound them into these with other threads, after which it drew threads over the edges of another part. By thus going round the whole rim of the opening, and by carrying and fixing the grains of earth in the threads which were the last stretched over the opening, it rendered its diameter smaller and smaller."
It was by working with its head that our mason gave to the new wall of its cocoon the necessary curvature. It was interesting to know how, as it could no longer put out its head, it could stop up the orifice.
"It knew how to change its manoeuvres. When the opening was reduced to a circle of only a few lines in diameter, it drew threads from a point on the circumference to another on the other side.... Thus the opening was covered in with a rather open network.... As soon as this web was finished, it got a grain of earth (which it had laid by until it was wanted), brought it up, placed it against the web, and by pushing and pressing it, made it pass through the web until it reached the exterior, and so in succession the whole of the web was covered with grains of earth.... It was not satisfied with rendering the exterior of this place exactly like the rest of the shell; it fortified it thoroughly; it added to it, one after another, layers of grains of earth, till it was as solid and as thick as the rest."
The larva of _Pyralis corticalis_, which is found on oak trees in the month of May, shows to what a point these little insects carry their industry in the construction of their cocoons, in the choice of their materials, in their manner of working them up, and in the forms they cause them to assume. Réaumur one day saw this caterpillar on a small branch, between two triangular appendages (Figs. 122, 123). This was the beginning of a cocoon. Each triangular blade was composed of a great number of small, thin, rectangular plates, taken from the bark of the twig. The caterpillar detached with its jaws a small band of bark, and fitted it on, and adjusted it with admirable precision against the edge already formed. It then fixed it securely with silk threads. Réaumur saw this caterpillar work and erect in this way a large blade during an hour and a half.
"When one sees," he says,[39] "an insect which, to construct a cocoon, begins by collecting together an infinite number of small plates of bark in order to compose of them two flat triangular blades; which, to gain its end, takes means that seem so roundabout, although they are the most suitable and the quickest it could adopt, one is very much tempted to consider such an insect, when one sees it thus acting, possessed of reason."
[39] Mem. 12, vol. i., p. 487.
These two blades are at last transformed into a regular cocoon. The little animal, which is at the same time architect, cabinet-maker, and weaver, arranges it in such a way as to form a hollow cone, which it only remains for it to shut. Réaumur calls this sort of cocoon or shell, _la coque en bateau_, the boat-shaped cocoon. Some caterpillars weave cocoons of the same form with pure silk.
To bring this subject to an end, we will mention the industry of the Puss Moth (_Dicranura vinula_), and that of a small _Tinea_, which eats the barley stored away in our granaries.
The larva of the puss moth employs in the construction of its shell the wood of the tree on which it has lived. It bites it up, and mixing it with a glutinous fluid which it secretes from its mouth, reduces it to a sort of paste, which it then uses in the formation of an envelope, of such hardness that a knife can hardly cut into it.
The _Tinea_ lines the interior of a grain, of which it has previously devoured the contents, with a coating of silk, and divides it thus into two different chambers. In one of these it is to change into a pupa; in the other it places its excrement. And so the little careful architect constructs its house in such a manner as to find in it tranquillity, cleanliness, and comfort.
When caterpillars have not within their reach the materials they are in the habit of employing, like good workmen, they content themselves with what they can get. Réaumur reared a caterpillar which formed its cocoon of pieces of the paper of which the box was made in which it was imprisoned.
What an extraordinary condition! what a strange phase of vitality does the chrysalis present to us--a being occupying the middle state between the caterpillar and the perfect insect! How little does it resemble that which it previously was, and what it will become! In appearance it is scarcely a living being; it takes no nourishment, and has no digestive organs; can neither walk nor drag itself along, and hardly bends the joints of its body. The outside skin of the chrysalis appears to be cartilaginous; it is generally smooth, although some species have hairs scattered over their bodies.
We distinguish in chrysalides two opposite sides. The one is the insect's back, the other its under side. On the upper part of the latter (Fig. 124) we perceive various raised portions, formed and arranged like the bands round the heads of mummies; the back is plain and rounded in a great number of pupæ; but a great many others have on the upper part, along the edges which separate the two sides, little humps, eminences broader than they are thick, ending in a sharp point (Fig. 125).
The head of the angular pupæ terminates sometimes in two angular parts, which diverge from each other like two horns (Fig. 126). In some other cases they are curved into the form of a crescent. These appendages sometimes give to the pupa the appearance of a mask, especially as an eminence placed on the middle of the back is rather like a nose, and the small cavities may represent the eyes (Fig. 125).
The colours of angular pupæ attract our attention. Some are superbly tinted; they appear to be wrapped in silk and gold. Others have only spots of gold and silver on their belly or their back. All, however, have not this remarkable splendour, not these metallic spots. Some are green, yellow, and spotted with gold. Generally they are brown. Réaumur has shown that this golden colour is not due, as was thought for a long while, to colouring matter, but to a little whitish membrane, placed under the skin, which reflects the light through the thin outer pellicle, in such a manner as to produce the optical illusion which imparts to the robe of the chrysalis the golden hues of a princess in grand costume. _All is not gold that glitters_, Réaumur proves literally, in the case of chrysalis.[40]
[40] The word is derived from [Greek: chrysos], golden; for that reason pupa is a better word than chrysalis, as this only strictly applies to a very small number; for the same reason aurelia is a bad word.--ED.
Let us add that the chrysalis remains thus superbly dressed as long as it is tenanted, but loses its colour as soon as the butterfly has quitted it.
The cone-shaped pupæ belong to the twilight and night-flying Lepidoptera, and to those butterflies whose caterpillars are onisciform, or in shape resembling a wood-louse. They are generally oval, rounded at the head, and more or less conical at the lower end. Their colour is generally of a uniform chestnut brown.
What a mystery is that which is accomplished in the transition from the chrysalis to the perfect state! Those great changes from the larva state to that of the pupa, and from the pupa to that of the imago, are accomplished with such rapidity, that the phenomena were looked on as sudden metamorphoses, like those related in mythology. It has been thought also that there was in these changes from one state to another a sort of resurrection. There is here neither sudden metamorphosis, nor, as we will show, resurrection. In fact, the chrysalis is a living being; it indeed shows its vitality by exterior movements. Under the old skin of a caterpillar about to moult, under the envelope which is soon to be cast off, the new integuments are being prepared.
Some days before the moult, split the caterpillar's skin, and you will find the skin which is to take its place already beneath. If, some days before the transformation of the caterpillar into a chrysalis, it is dissected, the rudiments of wings and antennæ may be discovered. If a chrysalis is examined on the outside, all the parts of the future insect can be distinguished under the skin: the wings, the legs, the antennæ, the proboscis, &c.; only, these parts are folded and packed away in such a manner that the chrysalis can make no use of them. It could not, moreover, make use of them on account of their incomplete development. Fig. 127 shows, after Réaumur,[41] a chrysalis magnified and seen from its lower side, on which we observe:--_a_, the wings; _b b_, the antennæ; _t_, the trunk or proboscis.
[41] Tome i., p. 382, planche 26, Fig. 6.
There is a moment when these parts, pressed one against each other, and as it were swathed up like a mummy, are very easily seen, for they are, as we may say, laid bare. This moment is that in which the pupa has just quitted the caterpillar's skin. It is then still soft and tender. Its body is moistened with a liquid, which, drying rapidly, becomes opaque, coloured, and of a membranous consistency. The result is that the parts which did not cohere in the least when the chrysalis made its first appearance, are fastened together, so that although they could at first be seen, through a layer of transparent fluid, they are now hidden under a sort of veil or cloak. It is necessary to seize then the moment of the birth of the chrysalis, to observe it accurately.
On examining the pupa before the liquid which pervades these parts has had time to dry, it resembles the perfect insect. All the exterior parts which belong to the imago can be distinguished. One recognises the head, which is then resting on the thorax; the two eyes and the antennæ (Fig. 128), which are brought forward like two ribbons; the wings also brought over the thorax, but these are separated artificially in the drawing we have given after Réaumur;[42] and lastly in the space left between the wings, the six legs, and the body of the insect.
[42] Tome i., p. 382, planche 26, Fig. 7.
To sum up: the pupa, when it approaches the period for being hatched is only a swaddled butterfly. Directly it has strength enough to rid itself of its wrapping, the insect frees itself from its fetters; it flies away, brilliant and free, and its many-coloured wings glitter in the sun.
The duration of the pupa state is variable, according to the species and the temperature. Réaumur placed in a hot-house, in the month of January, some pupæ which, in the ordinary course of things, would not have been hatched till the month of May, and a fortnight afterwards the imagos had appeared. On the other hand, he shut up some pupæ in an ice-house during the whole of a summer, and thus retarded their being hatched by a whole year. The influence of the temperature on the period of emerging, and, consequently, the influence of the seasons on the length of this period, are completely brought to light by these experiments.[43]
[43] They hardly seem from later experiments to be so fully explained. It is a well-known fact that many insects remain in this state a variable time--the Small Eggar (_Bombyx lanestris_) sometimes as many as seven years.--ED.
We will now see how the insect delivers itself from the last skin. To quit the pupa case is not so laborious an operation as it was for the same insect to quit the caterpillar's skin. This is because the pupa case is drier; it does not adhere to every part of the body, and is brittle. Those insects which are enclosed in a cocoon free themselves of the pupa envelope in the cocoon itself. To witness the last operation, the cocoon may be opened, and the pupa drawn out of it with care. If it is then placed in a box, the metamorphosis may be observed. To study this last evolution more at his ease, Réaumur covered a large extent of the wall of his study with pupæ of the _Vanessa polychloros_ and other species.
When the parts of the body of the insect have attained a certain degree of solidity within the envelope, it has no great difficulty in making the thin and friable membrane which surrounds it split in different places. If it even distends itself or moves, a small opening will be made in the dried skin. If the movements persist, the opening increases in size, and very soon allows the imago to emerge.
It is on the middle of the upper part of the thorax that the envelope begins to split. The split extends over the middle of the forehead and back. The pieces of the thorax open, separate themselves from the other parts to which they were fixed, and the insect can take advantage of the opening which is made, and escape. Little by little also it advances its head. The head is the first out of the old skin, and the insect sets itself entirely free.
This occupies rather a long time; for we must remember that, under the pupal envelope, its legs, its antennæ, its wings, and many other parts, are enclosed in special cases. These peculiar circumstances show that the animal has much trouble and must employ some time in setting free all the parts.
At last our prisoner has come out of its narrow cell, and is delivered from its old covering. What poet can describe to us the sensations of this charming and frail creature which has just risen from the tomb, and for the first time is enjoying the splendid light of day, the radiant sky, and the flowers redolent with delicious perfumes, which are inviting it to kiss and caress them!
The wings strike one most. They are very small at the time of birth.
Fig. 129 represents, after Réaumur,[44] a moth at the moment in which it has just emerged from the pupa. But at the end of a short period the wings become developed; only they are wrinkled, as Fig. 130, given by Réaumur, represents.
[44] Tome i., p. 654, planche 46, Fig. 1.
Réaumur having taken between his fingers a very short wing of a butterfly which was just hatched, drew it about gently in all directions. He succeeded thus in giving it the whole extent it would have assumed naturally. According to Réaumur the wing of a butterfly just born, which appears so small, is really already provided with all its parts, only it is folded and re-folded on itself. He supposes that what his hands did to lengthen the butterfly's wings, is done naturally by the liquids which are contained in the insect which has just emerged, and whose wings are no longer confined in their cases. At the time of its birth the wings are flat and thick; as they grow, little by little they spread themselves out and become curled up. When they are completely developed and flattened the wings become firm and hard imperceptibly, and this firmness extends at the same time to the whole of the body.
Figs. 131 and 132, borrowed, like the preceding, from the 14th Memoir of Réaumur (_sur la transformation des chrysalides en papillons_), show the states through which the wings of the same moth pass, before they are thoroughly developed.
Those pupæ enclosed in cocoons free themselves entirely or in part from their old skin, in the shell itself; but the imago is still a prisoner. It has broken through a first enclosure; it must open itself a way through the second. How does it manage to bore through the often very solid walls of this second prison, so as to regain its liberty? Réaumur states that in the Lackey Moth (_Bombyx neustria_) the head is the only instrument of which the insect makes use in opening a passage, the compound eyes then acting like files. These files cut the very fine threads of which the cocoon is composed, and as soon as the end of the cocoon is pierced through, the insect uses its thorax like a wedge, to enlarge the hole. It very soon manages to get its two front legs out, fixes itself by them on to the outside, and little by little emerges from its prison.
THE PERFECT INSECT.
Who does not admire the extraordinary splendour, the vivacity, the prodigious variety of colours of these brilliant inhabitants of the air? Some amateurs have devoted to the purchase of certain butterflies large sums of money. "Diamonds," says Réaumur on the subject, "have perhaps beauties no more real than those of a butterfly's wings; but they have a beauty which is more acknowledged by the world in general, and which is more recognised in commerce." The essential character of butterflies and moths makes them very easily recognisable among all other insects. All have four wings, which are covered with scales, that communicate to them the brilliant colours with which they are decorated. It is these scales which adhere to the fingers when we seize one of these charming creatures.
For a long time this dust was thought to be formed of very small feathers, but Réaumur showed that it is composed of little scales. Their form varies singularly, as we may see in Fig. 133, borrowed from the Memoirs of Réaumur,[45] which represents the different forms of the scales which cover the wings of Lepidoptera. M. Bernard Deschamps has closely studied them. According to this naturalist, they are composed of three membranes, or plates, superposed one on the other, of which the first is covered with granulations of a rounded form, which give to these scales their splendid and varied colours; the second plate is covered with silk, forming sometimes curious designs; the third plate, viz., that which is applied to the membrane of the wing, has the peculiar property of reflecting colours the most brilliant and the most varied, although the surface of the scales visible to the eye is often dull and colourless.
[45] Tome i., planche 7, Figs. 1 à 23.
"Supposing," says M. Bernard Deschamps, "that a painter was possessed of colours rich enough to represent on canvas with all their splendour, gold, silver, the opal, the ruby, the sapphire, the emerald, and the other precious stones which the East produces, that with these colours he formed all the shades which could result from their combination, one might affirm without the chance of contradiction, that he would have none of these colours and of their various shades, whatever might be the number, which could not be discovered by the microscope on part of the scales of the Lepidoptera, which Nature has been pleased to conceal from our gaze."
Each of these scales adheres to the membrane of the wing by a small tube, which is solidly fixed to it. Réaumur has called our attention to the admirable arrangement of these scales, which are disposed like those of fish, that is to say, in such a manner that those of a row shall partially overlap those in the following one.
In Fig. 134, representing a portion of the wing of the _Saturnia pavonia major_, magnified, which we borrow from Réaumur's Memoir, the scales are arranged in rows; isolated scales, and the points where other scales were fixed before they were taken off, are represented.
The membranous frame which supports the coloured scales of butterflies and moths is well worth a moment's consideration. It consists of two membranes intimately united by their interior surfaces, and divided into many distinct parts by horny, fistulous threads, more or less ramified, which seem intended to support the two membranes mentioned above, and which branch out from the base to the edge of the wing. Their number, counting from the exterior edge, is not always the same in the upper and lower wings. It varies from eight to twelve.
With its large and light wings, the butterfly can fly for a long time. But this flight is not in the least regular; it is not made in a straight line. When the insect has to go some distance, it flies alternately up and down. The line it takes is composed of an infinity of zig-zags, going up and down, and from right to left. This irregularity of flight saves the little insect from falling a prey to birds. "I saw one day with pleasure," says Réaumur, "a sparrow which pursued in the air a butterfly for nearly ten minutes without being able to catch it. The flight of the bird was nevertheless considerably more rapid than that of the butterfly, but the butterfly was always higher or lower than the place to which the bird flew, and where it thought it would catch it."
But let us leave the wings to pass on to the other parts of the butterfly. These other parts are the _thorax_ or chest, the body or _abdomen_, and the _head_.
The thorax is solidly put together, so as to bear the movements of the wings and legs. These latter are composed, as in other insects, of five parts: the coxa, the trochanter, the thigh, the shank, and the tarsus.
Many butterflies have all their six legs of equal length. In others, the two fore legs are very small, and are not suited for walking. In others, again, they are rudimentary, being deprived of hooks, very hairy, and applied on to the front of the breast.
This difference of structure may be seen in Figs. 135 and 136, one of which represents, after Réaumur, a leg unsuited for walking, very hairy, and terminated in a sort of brush resembling the tail on a tippet; and the other a leg furnished with hooks.
The abdomen has the form of an elongated, or--in the majority of species--an almost cylindrical oval. It is composed of five segments, each formed of an upper and a lower ring, joined together by a membrane. The first are larger than the others, and generally overlap the edges, which gives to this part of the body the power of dilating considerably.