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

by Unknown

It would be a great mistake to suppose that the insect, so soon as it leaves the pupa-case, is in that instant in every respect similar to what is known to us as the perfect insect. When, for example, a butterfly has just succeeded in extricating itself from its pupa-case, if we were to take it up in the expectation of finding it all at once decked with the glories of its wings, and elegant in its form, we should be disappointed. Owing to the cramped position which its limbs, and wings, and other organs, have so long been made to occupy, its appearance when it emerges from the pupa is necessarily different from that which it wears after all its limbs have been in free exercise, and the flutter of its wings has been heard over hill and dale, throwing the gentle air into an irregular line of tumult.

Now that summer has come, abundant opportunities for watching insects in all their stages may be had; and by careful searching of the garden, field, and woodland, a number of pupæ may be discovered on the very eve of disclosing the imago, or perfect insect. Let us suppose that the reader has not looked in vain; but has brought home in a tin box several of these singular objects, about which so much has been said. The pupa, ready to burst open and let loose the struggling captive within, is before him; the skin splits, the body of the insect appears, withdraws itself from the pupa-case and inner membranes, and the imago appears before him. But O, how different from the splendidly adorned insect which we know it is still to become! Its tender body is weak, soft, and languid, and bedewed with moisture. Its wings, instead of being of their usual size, and variegated with such glowing and admirably contrasted colours as only a Divine hand could create, are in the largest sized butterflies scarcely bigger than the nail of the little finger; instead of being uplifted in full strength and beauty, they hang drooping down over the sides of the trunk; and their colours are all dull, and muddy, and without any of those distinct spots and beautifully defined marks, streaks, and bands, which we observe with so much admiration in the wings of these insects. Altogether we might take it rather for a mutilated and imperfect insect, instead of what, in spite of its unattractive appearance, it really is,—a perfect insect, and in a short time to become a splendid one.

We have only to wait patiently for the lapse of a little time, and our desires will be fully gratified in beholding all these symptoms of weakness and imperfection disappear. The imago, attaching itself either to the cast-off pupa-case, or to some other convenient support, first stretches out one organ, and then another; its body loses the coating of moisture which bedewed it, its various parts become firm and hardened, and its colours come forth in all their beauty. All the parts which had been forced into a constrained position, now relieved, assume that which is natural to them in the perfect insect; and the wings no longer have a questionable appearance, but become expanded into those light and exquisite structures which form the peculiar beauty and characteristic of many insects, displaying themselves almost magically in the form, it may be, of the thin, transparent membrane of the fly, or as the painted tissue of the moth or butterfly, extending frequently to five or six times their previous dimensions.

Here let us take up, as an illustration of these beautiful and interesting phenomena, the concluding portion of the history of the dragon-fly, commenced at p. 302. The expansion of the wings of this elegant creature, after it has left the pupa-case, and fixed itself, generally on the stem of some pond plant, goes on so rapidly, that we can actually see them becoming larger each moment as we look upon them. If we were to attempt to trace their outline on paper, before the next stroke could be added, their form would be different. During the whole time that the development of the wings goes on, the insect continues perfectly still and immovable. Its wings are not stirred in the least, and the insect seems to avoid all risk of having these delicate organs touched by any surrounding object. Its wings, which are by-and-by to possess almost the stiffness of a thin layer of the mineral talc, are at present softer and more flexible than wet writing paper, and if they were to receive any injury while in this condition, that injury could never be repaired; hence the insect requires to be in a perfectly quiet condition. The dragon-fly seems to be fully aware of this, and in order to prevent the wings, as they increase in size, from drooping down towards, and coming in contact with, its body, it curves it in the manner represented, so as to allow for the expansion and elongation of the wings. At length, generally in a quarter of an hour's time, their development is fully completed; but they are not yet sufficiently firm and resisting to use in flying. If left to itself, the insect generally waits three, or even four hours, before it puts them into inactivity. In addition to the expansion and elongation of its wings, its body also elongates, until it has acquired its full dimensions. In vain should we look at first for the splendid colours in which these insects are decked; all the tints are blotted and unpleasing. Those charming species, which are spotted with beautiful blue and yellow tints intersprinkled with black, are, when they are but just emerged from the pupa, of a whitish yellow spotted with brown; the yellow marks, however, are seen to change to a beautiful deep orange, the brown to black, and some of the yellow marks to blue.

The following interesting description of the same changes as they take place in the butterfly is from the valuable work of Messrs. Kirby and Spence. "The pupa of a very interesting and beautiful butterfly, the only one of its description that Britain has yet been ascertained to produce, I mean Papilio Machaon, being brought to me by a friend early in May, this year, on the sixteenth of that month I had the pleasure of seeing it leave its pupa-case. 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, and particularly 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 to 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 germs 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."

If we were expert anatomists, and were armed with a good microscope, the study of the wing of a dragon-fly would prove one of great interest to us. These beautiful organs, however, must be examined when they are as yet but a little expanded, as in their completely developed state we should certainly fail in detecting their real structure. Thin and gauze-like as they seem, they are in reality double, consisting of an upper and a lower surface, between which are several parts which are necessary to preserve the shape and carry on the functions of the wings. The two surfaces of the wing enclose what are called the nervures, resembling in some respects the veins of a leaf. These are really fine hollow tubes of some firmness, which ramify in large numbers between the two membranes of the wing, and thus give strength and stability of figure to them. Along these tubes, or nervures, branches of the air-vessels, or tracheæ, and of the blood-vessels, run.

From these necessary details let us turn to make the difficult, but important inquiry—How are the wings expanded? by what means do they, from being thick, soft, and moist, become thin, hard, and dry? It is to be regretted that this inquiry cannot be answered with that satisfactory certainty which it deserves. It is very difficult to catch the insects in which the expansion of the wings is best observed at the right moment; and even then it is extremely difficult to say whether one cause more than another may contribute to this phenomenon. The following explanation of it is given by Messrs. Kirby and Spence, whose general accuracy on such subjects is well known. "As soon as the insect is disclosed, a fluid enters the tubes or nervures, 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, in many instances, 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. 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 the air was also of great importance to produce the expansion of the wing." But Swammerdam appears only twice to have seen the expansion of the dragon-fly, in which this phenomenon is best observed. Herold, an eminent naturalist, also attributes the expansion of the wing to the flow of an aqueous fluid into the nervures. M. Chabrier, a French entomologist, having observed a fluid in the interior of the nervures of the wings of insects, thinks it probable that they can introduce it into them and withdraw it at their pleasure, so as to facilitate their unfolding. When we call to mind the force with which the blow-fly, or flesh-fly, and the dragon-fly, are able to expand their heads by forcing air into them, we need scarcely ask for any other explanation than simply that the tracheæ are distended with air, and by that means the soft and yielding wings are made to assume their distended state.

It has been mentioned, that, in the case of the dragon-fly, the completion of the unfolding of its wings occupies about a quarter of an hour, but that sometimes it is even half an hour. The ordinary period is from five to ten or fifteen minutes in most insects, but it is sometimes prolonged to an hour, or to several hours. Again, in others, as we have already seen in the history of the emergence of the gnat, and other insects, from their aquatic state in the pupa, it is completed in a few seconds, and the insect only rests for this brief period on the surface of the water, previous to taking flight from its apparently somewhat dangerous position.