The Life of an Insect / being a history of the changes of insects from the egg to the perfect being.

by Unknown

Head of a Fly magnified. It shows opposite the letter x the remains of the membranous bag.

More singular still are the circumstances which mark the exit of the insect from the pupa case in other instances; and yet more strikingly than those narrated, do they exhibit to us the amazing exercise of the Divine attributes of wisdom and forethought in the case of these humble beings. The larva of a species of moth, which dwells in a wooden cell scooped out of the poplar tree, to which there is no door by which it can escape readily, gnaws away the wood until it leaves only an extremely delicate layer between it and the outside of the tree, which is as thin as writing paper. This done, it enters into the pupa state. Its time in that condition being accomplished, it moves itself by the same contrivance as the cossus, and actually pushes through the thin layer, and appears on the outside of the tree, thus making its escape from prison by pushing down a part of its prison wall!

An instance described by the naturalist Bonnet is yet more ingenious in the arrangements by which the insect escapes. While in the larva form it takes up its abode inside the leaf of an ash, curiously rolled up into a cone; and then, after a time, it becomes a pupa, forming a silken cocoon of a very slight texture, and, therefore, easily ruptured by the insect, which it suspends like a hammock in the midst of its habitation. 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. But, like the last-mentioned, this larva seems to be aware of the feebleness of its next condition, and gnaws in the leaf a round opening, taking care not to cut through the exterior thin layer of tissue, or epidermis. This door is to serve the insect for its exit in due time. But in proportion to its bulk, its green chamber is of considerable size. How, then, shall the insect know the exact place where its portal is situated? How, without a clue, shall it discover in its dark abode the precise circle which requires only a push to throw open its gate? Even this is foreseen and provided for. Out of all other positions in which the little hammock, of which we spoke, might have been hung, and they are numerous, the larva has been directed so to place it, that the silken cord which suspends the head is fastened close to the side of the door which it has previously constructed; and the insect, when it emerges from the pupa, guided by this thread, like Theseus, 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. Other insects adopt the same precaution of gnawing a doorway for the escape of the perfect insect, only leaving a sufficient thickness of outside tissue to protect the helpless pupa within from the invasion of enemies from without.

Cocoon of Emperor Moth.

Upon the pear or willow tree may sometimes be found an illustration of escape from the pupa, altogether well deserving our notice. In such situations the brown flask-shaped cocoon of the emperor moth may occasionally be discovered. In structure it is composed 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, 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, through which the moth makes its escape, the threads readily yielding to pressure from within, and acting somewhat on the principle of the wires of the opening to a rat-trap, or the willow cricks of an eel-trap. The silk of its cocoon is of so strong a texture, and so closely gummed, that had both ends been similarly closed, the egress of the insect would have been impracticable. But, it may be thought, such a cocoon is exposed to the attacks of a number of insect enemies, who might easily find entrance to it at the opening thus left at one of its ends. This source of peril has been foreseen. Within the exterior funnel-shaped end, at some little distance down, the insect has constructed a second funnel composed of a similar circle of needle-pointed threads, which, proceeding from the sides of the cocoon, converge to a point, and form a cone through which not the smallest aperture is left. From the arched structure of this singular dome, and from the fact just mentioned, that no visible opening can be discerned in it from without, it is rendered quite impenetrable to the most violent attacks of besiegers, while it yields to the slightest pressure from within, and allows the insect to emerge from its cocoon with the utmost facility. When it has passed through it, the elastic threads resume their former position, and the empty cocoon presents just the same appearance that it did before. A celebrated naturalist (Rösel), was sorely perplexed at this, the first time he had the gratification of watching the insect escape. He states that he could scarcely help thinking that there was something supernatural in the appearance of one of these fine emperor 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, and then found it to be empty!

Mr. Rennie mentions an instance, perhaps not so ingenious, but equally curious, with this history of the proceedings of the emperor moth, in a little insect, also a moth, which also dwells upon the willow. It spins an elastic shroud for its pupa, of the singular shape of a boat with the keel uppermost. Its first step is to spin two walls of whitish silk of the required form; and when these are completed, it draws them forcibly together with elastic threads, so placed as to retain them closely shut. The passage of the moth out of this cocoon might have struck Rösel with still greater surprise than he had felt at witnessing that of the emperor moth; for in that cocoon there was at least no apparent difficulty to prevent the egress of the insect, as the opening existed in it at one end, whereas in this there is no opening at all. The insect escapes at the joining of the sides, the threads giving way in a particular spot; and the sides, though originally requiring force to draw them together into the requisite form, become so elastic as to close again when the moth has passed between them, and made its escape. The cocoon preserves precisely the same form after the insect has quitted it as before, and it is impossible, by the naked eye, to detect the place of its exit.

As a general rule, insects make their escape from the pupa case head foremost; but there occurs a very singular exception in the case of some of the gall insects. The males of these insects contrive to make their escape out of the pupa case, formed of the dried skin, tail foremost; and as they thus back out of their dwelling, their wings are necessarily turned backwards over their heads; but a little exercise soon puts the ruffled insect in proper plume again, and the wings resume their customary position.

Some curiosity may be felt to know in what way the silk-worm moth escapes from the double prison,—the pupa case, and the cocoon,—in which she is concealed. How is the moth to make its way through the dense mass of fibres all glued together, which walls her in on every side? Her delicate wings and body would never endure anything like the severity of the struggle necessary to enable her to force her way through this, to her, solid and resisting mass. Though much attention has been paid to the transformations of this particular insect, it is somewhat curious that it is still a matter on which opinions are divided, as to how the insect succeeds in making its egress. Some suppose 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 file. Others hold the belief that the insect pours out a fluid which acts upon the gum and silken fibres of one end of the cocoon, and so softens them that they easily give way to the slightest pressure from within. "Perhaps the two opinions," observe Messrs. Kirby and Spence, "may be reconciled by supposing the silk-worm first to moisten, and then to break, the threads of the cocoon. In those that are of a slighter texture, a mere push against the moistened end is probably sufficient; and hence we find in so many newly-disclosed moths the hair in that part wet and closely pressed down."

It has been supposed, in cases where the cocoon is a hard, almost wooden cell, that the feeble insect prisoner within is provided with a peculiar chemical fluid, of greater powers of solution than are requisite in the last instance. The cocoon of the "pussmoth," in particular, is so hard and dense, as to resist even the point of a penknife; and the insect it holds confined within it is a weak creature, totally unprovided with any apparatus fit for penetrating walls so hard and dense as these. What is it to do? "Here," observe the writers last quoted, "the eyes are clearly incompetent; nor could any ordinary fluid assist their operations, for the gum which unites the woody particles of the cocoon is indissoluble in aqueous menstrua. What an aqueous solvent cannot effect, an acid is 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 pupa, 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 break down what was, a minute ago, a strong 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.[O] 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 can dissolve bone; and it is equally worthy of admiration. In both cases, the vitality of the membranous or fleshy receptacle secures it from the action of the included fluid; but how, who shall explain?"