Part 20

It is a general rule, that all winged insects pass through the larva and pupa state, before they assume their perfect form: there are also insects which have no wings, and yet undergo similar transformations, as the bed bug, the flea, &c. Other insects, which have no wings, and which always remain without them, never pass through the pupa state, but are subject to considerable changes, as well with respect to the number, as the figure of their parts; thus mites have four pair of feet, and two smaller ones at the fore part of the body, near the head; yet some of these are born with only three pair of feet, the fourth is not perceived till some time after their birth.[66] The figure of the monoculus quadricornis of Linnæus (Fauna Suecica, edit. Stockholm, 1761, No. 2049) changes considerably after its birth.[67] The julus is an insect with a great number of feet, some species having an hundred pair and upwards. M. De Geer has given a description of one with more than two-hundred pair,[68] and yet these at their birth have only three pair, the rest are not perceived till some time after.

[66] De Geer Memoires pour servir a l’Histoire des Insectes, tom. 1. p. 154.

[67] Ibid.

[68] Memoires des Scavans etrangers, tom. 3, p. 61.

OF THE CHANGE FROM THE LARVA TO THE PUPA STATE.

I shall now return to the caterpillar, and take notice of the care and provision it makes to pass from the larva state into that of the pupa or chrysalis; which is, in general, a state of imperfection, inactivity, and weakness, through which the insect, when it has obtained a proper size, must pass; and in which it remains often for months, sometimes for a whole year, exposed, without any means of escaping, to every event; and in which it receives the necessary preparations for its perfect state, and is enabled once more to appear upon the transitory scene of time. During its passage from one state to the other, as well as when it is in the pupa form, the microscopical observer will find many opportunities of exercising his instrument.

The transitions of the caterpillar from one state to another, are to it a subject of the most interesting nature; for in passing through them, it often runs the risk of losing its life, that precious boon of heaven, which is ever accompanied with a degree of delight, proportioned to the state in which the creature exists, and the use it makes of the gift it has received. If the caterpillar could therefore foresee the efforts and exertions it must make to put off its present form, and the state of weakness and impotence under which it must exist while in the pupa state, it would undoubtedly choose the most convenient place, and the most advantageous situation, for the performance of this arduous operation; one where it would be the least exposed to danger, at a time when it had neither strength to resist, nor swiftness to avoid the attack of an enemy. All these necessary instructions the caterpillar receives from the influence of an all-regulating Providence, which conveys the proper information to it by its own sensations: hence, when the critical period approaches, it proceeds as if it knew what would be the result of its operations. Different species prepare themselves for the change different ways, suited to their nature and the length of time they are to remain in this state.

When the caterpillar has attained to its full growth, and the parts of the future butterfly are sufficiently formed beneath its skin, it prepares for its change into the pupa state; it seeks for a proper place in which to perform the important business: the different methods employed by these little animals to secure this state of rest, may be reduced to four: 1. Some spin webs or cones, in which they inclose themselves. 2. Others conceal themselves in little cells, which they form under ground. 3. Some suspend themselves by their posterior extremity; 4. While others are suspended by a girdle that goes round their body. I shall describe the variety in these, as well as the industry used in constructing them, after we have gone through the manner in which the caterpillar prepares itself for, and passes through the pupa state.

Preparatory to the change, it ceases to take any food, empties itself of all the excrementitious matter that is contained in the intestines, voiding at the same time the membrane which served as a lining to these and the stomach. The intestinal canal is composed of two principal tubes, the one inserted into the other; the external tube is compact and fleshy, the internal one is thin and transparent; it is the inner tube, which lines the stomach and intestines, that is voided with the excrement before the change. It generally perseveres in a state of rest and inactivity for several days, which affords the external and internal organs that are under the skin an opportunity of gradually unfolding themselves. In proportion as the change into the pupa form approaches, the body is observed often to extend and contract itself; the hinder part is that which is first disengaged from the caterpillar skin; when this part of the body is free, the animal contracts and draws it up towards the head; it then liberates itself in the same manner from the two succeeding rings, consequently the insect is now lodged in the fore part of its caterpillar covering; the half which is abandoned remains flaccid and empty, while the fore part is swoln and distended. The animal, by strong efforts, still forcing itself against the fore part of the skin, bursts the skull into three pieces, and forms a longitudinal opening in the three first rings of the body; through this it proceeds, drawing one part after the other, by alternately lengthening and shortening, swelling and contracting the body and different rings; or else, by pushing back the exuvia, gets rid of its odious reptile form.

The caterpillar, thus stripped from its skin, is what we call the pupa, chrysalis, or aurelia, in which the parts of the future moth are inclosed in a crustaceous covering, but are so soft, that the slightest touch will discompose them. The exterior part of the chrysalis is at first exceedingly tender, soft, and partly transparent, being covered with a viscous fluid; this soon dries up, thickens, and forms a new covering for the animal, capable of resisting external injuries; a case, which is at the same time the sepulchre of the caterpillar, and the cradle of the moth; where, as under a veil, this wonderful transformation is carried on.

The pupa has been called a chrysalis, or creature made of gold, from the resplendent yellow colour with which some kinds are adorned. Reaumur has shewn us whence they derive this rich colour; that it proceeds from two skins, the upper one a beautiful brown, which lies upon or covers a highly polished and smooth white skin: the light reflected from the last, in passing through, gives it the golden yellow, in the same manner as this colour is often given to leather; so that the whole appears gilt, although no gold enters into the tincture. The chrysalis of the common white butterfly furnishes a most beautiful object for the lucernal opake microscope.

Those who are desirous to discover distinctly the various members of the moth in the pupa, should examine it before the fore-mentioned fluid is dried up, when it will be found to be only the moth with the members glued together; these, by degrees, acquire sufficient force to break their covering, and disengage themselves from the bands which confine them. While in this state, all the parts of the moth may be traced out, though so folded and laid together, that it cannot make any use of them; nor is it expedient that it should, as they are too soft and tender to be used, and pass through this state merely to be hardened and strengthened.

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.

OF THE IMAGO OR FLY STATE OF INSECTS.

As soon as the moth acquires sufficient strength to break the bonds which surround it, and of which it is informed by its internal sensations, it makes a powerful effort to escape from its prison, and view the world with new-formed eyes. The moth frees itself from the pupa with much greater ease than the pupa from the caterpillar; for the case of the pupa becomes so dry, when the moth is near the time of throwing off its covering, that it will break to pieces if it be only gently pressed between the fingers; and very few of the parts will be found, on examination, to adhere to the body. Hence, when the insect has acquired a proper degree of solidity, it does not require any great exertion to split the membrane which covers it. A small degree of motion, or a little inflation of the body, is sufficient for this purpose; these motions reiterated a few times, enlarge the hole, and afford the moth room to escape from its confinement. The opening through which they pass is always at the same part of the skin, a little above the trunk, between the wings, and a small piece which covers the head; the different fissures are generally made in the same direction. If the outer case be opened, it is easy to discover the efforts the insect makes to emancipate itself from its shell; when the operation begins, there seems to be a violent agitation in the humours contained in the little animal; the fluids seem to be driven with rapidity through all the vessels, and it is seen to agitate its legs, &c. as it were struggling to get free; these efforts soon break its brittle skin. The loosening the exterior bands of the pupa is not the only difficulty many moths have to encounter with; it has often also to pierce the cone or case in which it has been inclosed, and that at a time when its members are very feeble, when it is no longer furnished with strong jaws to pierce and cut its way through; but by the regular laws of divine order, means are furnished to every creature of attaining the end for which it was produced: thus, in the present case, some of these insects are provided with a liquor with which they soften and weaken the end of the cone; some leave one end feeble, and close it only with a few threads, so that a slight effort of the head enables the moth to burst the prison doors, and immerge into day.

When the moth first sees the day, it is humid and moist; but this humidity soon evaporates, the interior parts dry and harden as well as the exterior; the wings, which are wrinkled, being thick and small, then extend themselves, strengthen and harden insensibly, and the fibres which were at first flexible, become hard and stiff; so much so, that Malpighi considered them as bones: in proportion as these fibres harden, the fluid which circulates within them, and extends the wings, loses its force; so that if any extraneous circumstance prevent the motion of this fluid, at the first instant of the moth’s escape from its former state, the wings will then become ill-shaped; often expanding with such rapidity, that the naked eye cannot trace their unfolding. The wings, which were scarce half the length of the body, acquire in a few minutes their full size, so as to be nearly five times as large as they were before: nor is it the wings only which are thus increased; all their spots and colours, heretofore so minute as to be scarce discernible, are proportionably extended, so that what before appeared as only so many unmeaning and confused points, become distinct and beautiful ornaments; and those that are furnished with a tongue or trunk, curl and coil it up. When the wings are unfolded, the tongue rolled up, the moth sufficiently dried, and the different members strengthened, it takes its flight. Most of them, soon after they have attained their perfect state, void an excrementitious substance; Reaumur thinks that they eject very little, if any, during the rest of their lives.

In the progress of these insects, such changes take place, as we could have formed no conception of, if the great Author of these wonders had not been pleased to reward the industrious naturalist with the discovery.

If the moth be opened down the belly, and the unctuous parts which fill it, be removed, the gross artery, which has been called the heart, will be visible, and the contractions and dilatations, by which it pushes forward the liquor it contains, may be easily observed. One of the most remarkable circumstances is, that the circulation of this fluid in the moth is directly contrary to that which took place in the caterpillar; in this, the liquor moved from the tail to the head, whereas in the moth, it moves from the head to the tail; so that the fluid which answers the purposes of the blood in the moth, goes from the superior, towards the inferior parts, but in the voracious sensual caterpillar, the order is inverted, it proceeds from the inferior towards the superior parts; all its members, formerly soft, inactive, and folded up under an envelope, are expanded, strengthened, and exposed to observation.

The food of the caterpillar is gross and solid, and even this it is obliged to earn with much labour and danger; but, when freed as it were from the jaws of death, and arrived at its perfect form, the purest nectar is its potion, and the air its element. It was supplied with coarse food, in the first state, by the painful operation of its teeth, which was afterwards digested by a violent trituration of the stomach. The intestines are now formed in a more delicate manner, and suited to a more pure and elegant aliment, which nature has prepared for its use from the most fragrant and beautiful flowers. Many internal parts of the caterpillar disappear in the chrysalis, and many that could not be perceived before, are now rendered visible: the interior changes are not less surprizing than those of the exterior form, and are, properly speaking, creative of them; for it is from these the exterior form originates, and with these it always corresponds. In a word, the creature that heretofore crept upon the earth, now flies freely through the air; and far from creating our aversion by its frightful prickles and foul appearance, it attracts our notice by the most elegant shape and apparel, and, from being scarce able to move from one shrub to another, acquires strength and agility to tower far above the tallest inhabitant of the forest.

OF THE SILK-WORM.

The industry of those that spin cones or cases, in which they inclose themselves, in order to prepare for their transformation in security, is more generally known, as it is from one species of these that we derive so many benefits, namely from the silk-worm, whose works afford an ornament for greatness, and add magnificence to royalty. All caterpillars undergo similar changes with it, and many in the butterfly state greatly exceed it in beauty: but the golden tissue, in which the silk-worm wraps itself, far surpasses the silky threads of all the other kinds; they may indeed come forth with a variety of colours, and wings bedecked with gold and scarlet, yet they are but the beings of a summer’s day; both their life and beauty quickly vanish, and leave no remembrance after them; but the silk-worm leaves behind it such beneficial monuments, as at once record the wisdom of its Creator, and his bounty to man.[70]

[70] Pullein on the Culture of Silk.