But the most extraordinary motion of pupæ is jumping. In the year 1810 I received an account from a very intelligent young lady, who collected and studied insects with more than common ardour and ability, that a friend had brought her a chrysalis endued with this faculty. It was scarcely a quarter of an inch in length; of an oval form; its colour was a semitransparent brown, with a white opake band round the middle. It was found attached, by one end, to the leaf of a bramble. It repeatedly jumped out of an open pill-box that was an inch in height. When put into a drawer in which some other insects were impaled, it skipped from side to side, passing over their backs for nearly a quarter of an hour with surprising agility. Its mode of springing seemed to be by balancing itself upon one extremity of its case. About the end of October one end of the case grew black, and from that time the motion ceased; and about the middle of April, in the following year, a very minute ichneumon made its appearance by a hole it had made at the opposite end.—Some time after I had received this history, I happened to have occasion to look at Reaumur's Memoir upon the enemies of caterpillars, where I met with an account of a similar jumping chrysalis, if not the same. Round the nests of the caterpillar of the processionary moth, before noticed[463], he found numerous little cocoons suspended by a thread three or four inches long to a twig or a leaf, of a shortened oval form, and close texture, but so as the meshes might be distinguished. These cocoons were rather transparent, of a coffee-brown colour, and surrounded in the middle by a whitish band. When put into boxes or glasses, or laid on the hand, they surprised him by leaping. Sometimes their leaps were not more than ten lines, at others they were extended to three or four inches, both in height and length. When the animal leaps, it suddenly changes its ordinary posture (in which the back is convex and touches the upper part of the cocoon, and the head and anus rest upon the lower), and strikes the upper part with the head and tail, before its belly, which then becomes the convex part, touches the bottom. This occasions the cocoon to rise in the air to a height proportioned to the force of the blow. At first sight this faculty seems of no great use to an animal that is suspended in the air; but the winds may probably sometimes place it in a different and unsuitable position, and lodge it upon a leaf or twig: in this case it has it in its power to recover its natural station. Reaumur could not ascertain the fly that should legitimately come from this cocoon, for different cocoons gave different flies: whence it was evident that these ichneumons were infested by their own parasite[464]. This might be the case with that of the lady just mentioned. Perhaps, properly speaking, in this last instance the motions ought rather to be regarded as belonging to a larva; but as it had ceased feeding, and had inclosed itself in its cocoon, I consider it as belonging to the present head.
You may probably here feel some curiosity to be informed how the numerous larvæ that are buried in their pupa state, either in the heart of trees, under the earth, or in the waters, effect their escape from their various prisons and become denizens of the air, especially as you are aware that each is shrouded in a winding-sheet and cased in a coffin. In most, however, if you examine this coffin closely, you will see resurgam written upon it. What I mean is this. The puparium or case of the animal is furnished with certain acute points (adminicula) generally single, but in some instances forked, looking towards the anus, and usually placed upon transverse ridges on the back of the abdomen, but sometimes arming the sides or the margins of the segments. By this simple contrivance, aided by new-born vigour, when the time for its great change is arrived, the included prisoner of hope, if under ground, pushes itself gradually upwards, till reaching the surface its head and trunk emerge, when an opening in the latter being effected by its efforts, it escapes from its confinement, and once more tastes the sweets of liberty and the joys of life. Those that are inclosed in trees and spin a cocoon, are furnished with points on the head, with which they make an opening in the former. The pupa of the great goat-moth (Cossus ligniperda) thus, by divers movements, keeps disengaging itself from this envelope, till it arrives at a hole in the tree which it had made when a caterpillar; when its anterior part having emerged, it stops short, and so escapes a fall that might destroy it. After some repose, in consequence of very violent efforts, the puparium opens, and it escapes from its prison[465].
The insects of the Trichoptera order, or case-worm flies are quiescent when they first assume the pupa, but become locomotive towards the close of their existence in that state. Since they inhabit the water when they become pupæ, Providence has furnished them with the means of quitting that fluid without injury, when they are to exchange it for the air; which in their winged state is their proper sphere of action. I have before described to you the grates which shut up their cases when they became quiescent[466]; if they had no means of piercing these grates, they would perish in the waters. The head of these pupæ is provided at first with a particular instrument, which enables them to effect this purpose; its anterior part is armed with a pair of hooks in form resembling the beak of a bird; and with this, previously to their last change, they make an opening in the grate which, though it once defended, now confines them. But at this moment, perhaps, the insect has a considerable space of water to rise through before she can reach the surface. This is all wisely provided for; before she leaves the envelope which covers her body, she emerges from the water, and fixes herself upon some plant or other object, the summit of which is not overflowed. But you will here, perhaps, ask—How can a pupa in her envelope, with all her limbs set fast, do this? This affords another instance of the wise provision of the beneficent Father of the universe for the welfare of his creatures. The antennæ and legs of this tribe of insects, when they are pupæ, are not included, as is the case with most that are quiescent in that state, in the general envelope; but each in a separate one, so as to allow it free motion. Thus the insect when the time is come for its last change can use them (except the hind-legs, which being partly covered by the wing-cases remain without motion) with ease. It then stretches out its antennæ, and steering with its legs makes for the surface. De Geer saw one just escaped from its case run and swim with surprising agility over the bottom of a saucer, in which he had put some cases of these flies; and at last when he held a piece of stick to it, it got upon it, and having emerged from the water, prepared to cast its envelope. It is remarkable, that the envelope of the intermediate tarsi, like the posterior ones of Dytisci, is fringed on one side with hairs, to enable the insects to use them as swimming feet[467], while those neither of the larva nor imago are so circumstanced.
I am, &c.
[LETTER XXIII.]
MOTIONS OF INSECTS. (Imago.)
III. The motions of insects in their perfect or imago state are various, and for various purposes; and the provision of organs by which they are enabled to effect them is equally diversified and wonderful. It will be convenient to divide this multifarious subject; I shall therefore consider their motions under two principal heads:—motions of insects reposing—and motions of insects in action;—and this last head I shall further subdivide into motions whose object is change of place, and sportive motions.