The butterfly guide

by W. J. Holland

Plate C

When the caterpillar emerges from the egg the head is in many cases seen to be very large in proportion to the rest of the body. This relative disparity soon disappears, however, as growth takes place. As the larva increases in size, it soon reaches a point at which the skin in which it made its appearance in the world becomes too small and tight for further comfort and use. Thereupon it proceeds to moult, or shed, this now uncomfortable garment. The skin splits along the back and the caterpillar crawls out of it. Before moulting the caterpillar always takes the precaution to attach this outer skin by strands of silk to the leaf or branch upon which the moult is to take place. Having escaped from the cast-off garment, the caterpillar sometimes turns around and eats it before resuming its vegetable diet. The process of moulting takes place four or five times before the larva changes into a chrysalis.

Caterpillars differ entirely from butterflies in that they are able to produce silk. Silk is a viscous fluid secreted by elongated sacs located in the dorsal region. These sacs communicate with a minute tube-like organ, known as the spinneret, which is located on the under side of the head of the caterpillar, just back of the mandibles. The fluid silk, as it is ejected through the spinneret, immediately hardens on contact with the air and is deposited in the form of very fine threads or filaments which the caterpillar uses for various purposes, sometimes as lines with which to guide itself from place to place and enable it to retrace its steps to its favorite resting-place, sometimes to tie together the leaves in which it forms its nest, or to weave a sort of shelter in which it conceals itself, and finally to make the little buttons and the girdles by which, as we shall see later, the chrysalis is held in place. Many moths weave from silk compact structures known as cocoons, in which the chrysalis is lodged. Butterflies do not weave true cocoons.

The time spent by the insect in the egg is generally short. The time passed in the larval state may be short or long. When butterflies hibernate, or pass the winter, as caterpillars, the time spent in this state is long, and especially in the case of those species which inhabit arctic regions. There are some butterflies which occur north of the Arctic Circle, and we have ascertained that these, because the summers are so short in the far north, pass two summers and the intervening winter in the larval condition, and another winter in the pupal stage, before they emerge and take wing. On the other hand, under more temperate conditions butterflies of certain species may produce two or even three broods in a summer, and in subtropical or tropical lands there may be even more broods produced.

While it is true that almost all the larvæ of lepidoptera subsist upon vegetable food, there are nevertheless exceptions, one of which is that of the Harvester, Feniseca tarquinius (see [Plate LXXXVIII], Fig. 1), the slug-like larva of which feeds upon scale-insects, or mealy bugs, sharing the habit with certain allied species which occur in Africa and Asia, as the writer has had occasion to point out a number of years ago in certain of his writings.

THE PUPA, OR CHRYSALIS

The third stage in the life of lepidoptera is known as the pupal stage. The caterpillar, having undergone successive moults and reached maturity, is transformed into a pupa, or chrysalis (see [Plate C], Figs. d and e). From having been an active, worm-like creature, greedily feeding upon its appropriate food, it reverts to a form which is stationary, as was the egg, and ceases to have the power of locomotion. An examination of the structure of all chrysalids shows that they contain an immature butterfly. The segments of the chrysalids enclose the corresponding segments of the body of the butterfly, and in sheathing plates of chitinous matter are enfolded the wings and all the other organs which are necessary to the life of the butterfly when it shall have emerged and taken wing. The act of transformation from the caterpillar stage to the pupal stage is very wonderful. The caterpillar makes provision for the great change by weaving a little button of silk and, in the case of many of those larvæ, the chrysalids of which are not pendant, by also weaving a little girdle of silk, which it passes around its back, and which holds it in place very much as an Indian baby is held by the strap which passes over the shoulders of the squaw (see [Plate C], Fig. e). Having made these preliminary arrangements the caterpillar becomes very quiet, its hind pro-legs being securely hooked and tangled into the silken button to which it is attached. After a while, when the proper moment has arrived, the skin of the caterpillar splits, just as in the moults which preceded, and by a series of wriggling or vibratory motions the chrysalis succeeds in working off the skin of the caterpillar until it has all been shed except where near the end of the abdomen the skin is caught between the edges of two of the horny rings which form the abdomen. Then the insect with the cremaster, as it is called, a little spikelet at the very tip of the chrysalis, which is armed with small hooks, proceeds to feel about until these hooks become entangled in the silk of the button which has been provided on the under surface of the twig, the stone, or the fence rail, where the transformation is occurring. As soon as the chrysalis is securely hooked into the button of silk it lets go of the little section of the skin by which it has been supported and rapidly assumes the shape in which it will remain until the time of its emergence as a butterfly. These changes are illustrated on [Plate C], Figs. a-d, which are reproduced from “The Butterfly Book” after the drawings of the late Prof. C. V. Riley. The chrysalids of all the Nymphalidæ are pendant; those of the other families, except the Hesperiidæ, are provided with girdles, as is shown on [Plate C], in the figures which represent the chrysalis of Papilio philenor. The chrysalids of the Hesperiidæ, like the chrysalids of moths, are either formed in loosely woven coverings of leaves tacked together with silken threads, or lie free under leaves and rubbish upon the ground, thus resembling the chrysalids of moths.

Chrysalids are for the most part rather obscure in coloring, though some are quite brilliantly marked with metallic spots as in the case of the common Milkweed Butterfly, Anosia plexippus, the chrysalis of which is pearly green in color, ornamented with bright golden spots.

The forms assumed by chrysalids are very various, especially among the Nymphalidæ, and they are often ornamented with curious projections and tubercles, imparting to them very odd outlines.