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Their most usual shape is elongated and almost cylindrical, or slightly tapering at one or both ends. Of these, some are smooth, or only studded with short down or hairs; such are the caterpillars of the Swallow-tail butterfly (fig. 1), of the Brimstone (fig. 2), Clouded Yellows, and Garden, and other white butterflies. Others, of the same general form, are beset with long branched spines, making perfect chevaux-de-frise; such are those of the Peacock, Red Admiral, Painted Lady, and the Silvery Fritillaries.
The caterpillars of another large section have the body considerably thicker in the middle (rolling-pin shaped), and the tail part two-forked, or bifurcate. This form belongs to the numerous family that includes the Meadow-brown (fig. 3), the Ringlets, and many others.
The bizarre personage, at fig. 4, turns to the graceful White Admiral butterfly.
The Purple Emperor begins his royal career in the curious form shown at fig. 5—a shape unique among British butterflies, as beseems that of their sovereign; and he carries a coronet on his brow already.
All those beautiful little butterflies called the Hair-streaks (fig. 9), the Blues (fig. 10), and the Coppers, have very short and fat caterpillars, that remind one forcibly of wood-lice—a shape shared also by that small butterfly with a big name, the Duke of Burgundy Fritillary (fig. 8), an insect very distinct from the Fritillaries above mentioned with thorny caterpillars.
The legs of a caterpillar are usually sixteen in number, and composed of two distinct kinds, viz. of six true legs, answering to those of the perfect insect, and placed on the foremost segments of the body; and of ten others, called "prolegs;" temporary legs, used principally for strengthening the creature's hold upon leaf or branch.
Like the rest of its body, the caterpillar's head widely differs in structure from that of the perfect insect, being furnished with a pair of jaws, horny and strong, befitting the heavy work they have to get through, and shaped like pincers, opening and shutting from side to side, instead of working up and down after the manner of the jaws in vertebrate animals. This arrangement offers great convenience to the creature, feeding, as it is wont to do, on the thin edge of a leaf. It is a curious sight to watch a caterpillar thus engaged. Adhering by his close-clinging prolegs, and guiding the edge of the leaf between his forelegs, he stretches out his head as far as he can reach, and commences a series of rapid bites, at each nibble bringing the head nearer the legs, till they almost meet; then stretching out again the same regular set of mouthfuls is abstracted, and so on, repeating the process till a large semi-circular indentation is formed, reaching perhaps to the midrib of the leaf; then shifting his position to a new vantage ground, the marauder recommences operations, another sweep is taken out, then another, and soon the leaf is left a mere skeleton.
But a change, far more important than mere skin-shifting, follows close upon the animal's caterpillar-maturity, complete as soon as it ceases to grow.
The form and habits of a worm are to be exchanged for the glories and pleasures of winged life; but this can only be done at the price of passing through an intermediate state; one neither of eating, nor of flying, but motionless, helpless and death-like.
This is called the Chrysalis or Pupa state.
Pupa is a Latin word, signifying a creature swathed, or tied up; and is applied to this stage of all insects, because all, or some, of their parts are then bound up, as if swathed.
The term Chrysalis is applicable to butterflies only, and, strictly, only to a few of these—Chrysalis[1] being derived from the Greek χρυσός (chrysos), gold—in allusion to the splendid gilding of the surface in certain species, such as the Vanessas, Fritillaries, and some others.
In the older works on entomology we frequently meet with the term Aurelia applied to this state, and having the same meaning as chrysalis, but derived from the Latin word Aurum, gold.
Here the reader is again referred to Plate I. for a series of the principal forms assumed by the chrysalides of our native butterflies, and as these for the most part represent the next stage of the caterpillars previously figured, an opportunity is afforded of tracing the insect's form through its three great changes; the whole of the butterflies in their perfect state being given in their proper places in the body of the work.
The complicated and curious processes by which various caterpillars assume the chrysalis form, and suspend themselves securely in their proper attitudes, have been most accurately and laboriously chronicled by the French naturalist, Réaumur; but his memoirs on the subject, which have been frequently quoted into the larger entomological works, are too long for insertion here in full, and any considerable abbreviation would fail to convey a clear idea of the process, on account of the intricacy of the operations described. So I can only here allude to the difficult problems that the creature has to solve, referring the reader to the above-mentioned works for a detailed description of the manner of doing so; or, better still, I would recommend the country resident to witness all this with his own eyes. By keeping a number of the caterpillars of our common butterflies, feeding them up, and attentively watching them when full-grown, he will now and then detect one in the transformation act, and have an opportunity of wondering at the curious manœuvres of the animal, as it triumphs over seeming impossibilities.
By reference to the figures of chrysalides on Plate I. it will be seen that there are two distinct modes of suspension employed among them; one, by the tail only, the head hanging down freely in the air:—in the other, the tail is attached to the supporting object; but the head, instead of swinging loosely, is kept in an upright position by being looped round the waist with a silken girdle.
To appreciate the difficulty of gaining either of the above positions, we must bear in mind that, before doing so, the caterpillar has to throw off its own skin, carrying with it the whole of its legs, and the jaws too—leaving itself a mere limbless, and apparently helpless mass—its only prehensile organs being a few minute, almost imperceptible hooks on the end of the tail; and the required position of attachment and security is accomplished by a series of movements so dexterous and sleight-of-hand like, as to cause infinite astonishment to the looker-on, and, as Réaumur justly observes, "It is impossible not to wonder, that an insect, which executes them but once in its life, should execute them so well. We must necessarily conclude that it has been instructed by a Great Master; for He who has rendered it necessary for the insect to undergo this change, has likewise given it all the requisite means for accomplishing it in safety."
If we examine a chrysalis we are able to make out, through the thin envelope, all the external organs of the body stowed away in the most orderly and compact manner. The antennæ are very conspicuous, folded down alongside of the legs; and precisely in the centre will be seen the tongue, unrolled and forming a straight line between the legs. The unexpanded wings are visible on each side—very small, but with all their veinings distinctly seen; and the breathing holes, called spiracles, are placed in a row on each side of the body.
The duration of the chrysalis stage, like that of the egg, is extremely variable, and dependent on difference of temperature. As an instance of this, one of our common butterflies has been known to pass only seven or eight days in the chrysalis state; this would be in the heat of summer. Then, in the spring, the change occupies a fortnight; but when the caterpillar enters the chrysalis state in the autumn, the butterfly does not make its appearance till the following spring. Furthermore, it has been proved by experiment, that if the condition of perpetual winter be kept up by keeping the chrysalis in an icehouse, its development may be retarded for two or three years beyond its proper time; while, on the other hand, if in the middle of winter the chrysalis be removed to a hothouse, the enclosed butterfly, mistaking the vivifying warmth for returning summer, makes its début in ten days