To examine the moth concealed under the skin of a caterpillar, one of them should be taken at the last change; when the skin begins to open, it should be drowned in spirit of wine, or some strong liquor, and be left therein for some days, that it may take more consistency and harden itself; the skin of the caterpillar may then be easily removed: the chrysalis, or feeble moth, will be first discovered, after which the tender moth may be traced out, and its wings, legs, antennæ, &c. may be opened and displayed by an accurate observer.
The parts of the moth or butterfly are not disposed exactly in the same manner in the body of the caterpillar, as when left naked in the chrysalis. The wings are longer and narrower, being wound up into the form of a cord, and the antennæ are rolled up on the head; the tongue is also twisted up and laid upon the head, but in a very different manner from what it is in the perfect animal, and different from that which it lies in within the chrysalis; so that it is by a progressive and gradual change, that the interior parts are prepared for the pupa and moth state. The eggs, hereafter to be deposited by the moth, are also to be found, not only in the chrysalis, but in the caterpillar itself, arranged in their natural and regular order.
While in this state, the creature generally remains immoveable, and seems to have no other business but patiently to attend the time of its change, which depends on the parts becoming hard and firm, and the transpiration of that humidity which keeps them soft; the powers of life are as it were absorbed in a deep sleep; the organs of sensation seem obliterated, being imprisoned by coverings more or less strong, the greater part remains fixed in those situations which the caterpillar had selected for them till their final metamorphosis; some, however, are capable of changing place, but their movements are slow and painful.
The time, therefore, which the moth or butterfly remains in the pupa state is not always the same, varying in different species, and depending also upon the warmth of the weather, and other adventitious circumstances; some remain in that situation for a few weeks; others do not attain their perfect form for eight, nine, or eleven months: this often depends on the season in which they assume the pupa form, or rather on the time of their birth. Some irregularities are also occasioned by the different temperature of the air, by which they are retarded or accelerated, so as to be brought forward in the season best suited to their nature and the ends of their existence. I have heard of an instance, where the pupa, produced from caterpillars of the same eggs, nourished in the same manner, and which all spun up within a few days of each other in the autumn, came into the fly state at three different and distant periods; viz. one-third of them the spring following their change, one-third more the succeeding spring, and the remainder the spring after, making three years from their first hatching; a further and manifest proof of the beauty and wisdom of the laws of Divine order, which are continually operating for the best interests of all created beings. As the transformation of insects is retarded by cold, and accelerated by heat, the ordinary period of these changes may sometimes be altered, by placing them in different degrees of heat or cold; by these they may be awakened sooner to a new state of existence, or kept in one of profound sleep.[69]
[69] Reaumur Memoires sur les Insectes, tom. 2, mem. 1.
There are some caterpillars which remain in their cone eight or nine months before they acquire the complete pupa state; so that their duration in that form is much shorter than it naturally appears to be.
OF THE PREPARATION OF THE CATERPILLAR FOR THE METAMORPHOSES.
The industry of the caterpillar, in securing itself for its change into the chrysalis, must not be passed by; not only because it naturally leads the reader to consider and admire that divine agency, by which the insect is informed, but because the different modes it makes use of cannot be properly investigated, without the assistance of glasses, it therefore consequently becomes a proper subject for the microscope; we shall select from a great variety, a few instances, to animate the reader in these researches.
Some caterpillars, towards the time of their change, suspend themselves from the branch of a tree, with the head downwards; in this position they assume the pupa form, and from thence immerge a butterfly or moth. In order to secure itself in this position, the insect covers with threads that part of the branch from which it means to suspend itself; it places these in different directions, and then covers them with other threads, laying on several successive thicknesses, each new layer being smaller in size than that which preceded it; forming, when finished, a little cone or hillock of silk, as will be found when examined by the microscope. The caterpillar hooks itself by the hinder feet to this hillock, and when it has found by several trials that it is strongly fixed thereto, throws itself forward, letting the body fall with the head downwards. Soon after it is thus suspended, it bends the fore part of the body, keeping this bent posture for some time, then straitening the body, again in a little time bending it, and so on, repeating this operation till it has formed a slit in the skin upon the back; part of the pupa soon forces itself through this, and extends the slit as far as the last crustaceous feet; the pupa then forces upwards the skin, as we would push down a stocking, by means of its little hooks and the motion of the body, till it has slipped it off to that part from which the caterpillar had suspended itself. But the pupa has still to disengage itself from this small packet, to which the exuvia is now reduced: here the observer will find himself interested for the little animal, anxious to learn how the pupa will quit this skin, and how it will be enabled to fix itself to the hillock, as it has neither arms nor legs. A little attention soon explains the operation, and extricates the observer from his embarrassment. It seizes the exuvia by the rings of the body, and thus holds itself as it were by a pair of pincers; then, by bending the tail, it frees itself from the old skin, and by the same method soon suspends itself to the silken mount; it lengthens out the hinder part of the body, and clasps, by means of its rings, the various foldings of the exuvia, one after another; thus creeping backward on the spoils, till it can reach the hillock with the tail; which, when examined by the microscope, will be found to be furnished with hooks to fix itself by. It is surprizing to see with what exactness and ease these insects perform an operation so delicate and dangerous, which is only executed once in their lives; and nought else can account for it, but the consideration that HE, who designed that the caterpillar should pass through these changes, had provided means for that end, regularly connecting the greater steps by intermediate ones, the desire of extending their species forming and acting upon the organization, till the purposes of their life are completed. Different kinds of these insects require variety in the mode of suspension; some fix themselves in an horizontal position, by a girdle which they tie round their body; this girdle appears to the naked eye as a single thread; when examined with the microscope, it will be found to be an assemblage of fine threads, lying close to each other, so fixed as to support the caterpillar, and yet leave it in full freedom to effect the changes. Like the preceding kind, it fixes the girdle to the branch of a tree; in this situation it remains for some time motionless, and then begins to bend, move, and agitate its body in a very singular manner, till it has opened the exterior covering, which it pushes off and removes much in the same manner as we have described in the preceding article, and yet with such dexterity, that the pupa remains suspended by the same girdle.