Читать книгу The Butterflies of the British Isles - Richard South - Страница 8

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The second stage is that of the caterpillar, and in some species, such as the Red Admiral, this is of very short duration, a few weeks only, whilst in others, as for example the Small Blue, it usually lasts for many months. There is considerable diversity both in the shape and, where it is present, in the hairy or spiny clothing (armature) of caterpillars. All, however, are alike in one respect, that is the body is divided into thirteen more or less well-defined rings (segments), which together with the head make up fourteen divisions. In referring to these body-rings, the first three nearest the head, each of which is furnished with a pair of true legs (thoracic legs), are called the thoracic segments, as they correspond to the thorax of the perfect butterfly. The remaining ten rings are the abdominal segments; the last two are not always easily separable one from the other, and so for all practical purposes they may be considered only nine in number. These nine rings, then, correspond to the abdomen of the future butterfly. The third to sixth of this series have each a pair of false legs (prolegs), and there is also a pair on the last ring; the latter are the anal claspers.

The warts (tubercles) are the bases of hairs and spines, and are to be seen in most butterfly caterpillars, but they generally require a lens to bring them clearly into view. These warts are usually arranged in two rows on the back (dorsal series) and three rows on each side (lateral series).

Fig. 1.

Young caterpillar of Orange-tip highly magnified.

(After Sharp.)

All the various parts referred to, or to be presently mentioned, may be seen in Fig. 1, which also shows a peculiarity that is found in very young caterpillars of the Orange-tip, and in some others of the "Whites" (Pieridæ). The odd thing about this baby caterpillar is that the fine hair arising from each wart is forked at the tip (Fig. 1, a), and holds thereon a minute globule of fluid. When the caterpillars become about half grown these special hairs are lost in a general clothing of fine hair. Fig. 1, b, represents a magnified single ring of the caterpillar, and this shows a spiracle and the folds of the skin (subsegments). The manner in which such folding occurs is to be observed in the higher study of larval morphology.

On each ring, except the second (including now the three thoracic with the nine abdominal; and so making twelve rings), the third, and the last, there is an oval or roundish mark which indicates the position of the breathing hole (spiracle). Through these minute openings air enters to the breathing tubes (tracheæ), which are spread throughout the interior of the caterpillar in a seemingly complicated kind of network of main branches and finer twigs; air is thus conveyed to every part of the body. In the event of one or two air-holes becoming in any way obstructed, the caterpillar would possibly be none the worse; but if all the openings were closed up effectually, it would almost certainly die. Total immersion in water, even for some hours, is not always fatal.

Turning again to the "feet" of the caterpillar, it will be seen from the figure that the true legs (a) differ from the false legs (b) in structure. The former are horny, jointed, and have terminal claws; the latter are fleshy, with sliding joints, and the foot is furnished with a series of minute hooks which enable the caterpillar to obtain a secure hold when feeding, etc. The false legs are also the chief means of locomotion, as the true legs are of little service for this purpose. The true legs, however, appear to be of use when the caterpillar is feeding, as the leaf is held between them so as to keep it steady whilst the jaws are doing their work.

Fig. 2.

(a) True and (b) false legs.

In the accompanying figure of the head of a caterpillar the mouth parts are clearly shown. The biting jaws (mandibles) are slightly apart, above them is seen the upper lip (labrum), and below them is the under lip (labium or lingua). The maxillæ are very tiny affairs, but they should be noted because in the butterfly they become the basal portions of the two tubes which, when united together, form the sucking organs (proboscis). The eyes, or ocelli as they are termed, are minute, and are said to be of slight use to the caterpillar as organs of sight, so that it probably has to depend on its little feelers (antennæ) for guidance to the right plants for its nourishment. Attention should also be given to the spinneret, as it is by means of this that the silken threads, etc., for its various requirements are provided; the substance itself being secreted in glands placed in the body of the caterpillar. The palpi are organs of touch, and seem to be of use to the caterpillar when moving about.

Fig. 3.

a, labrum; b, mandible; c, antenna; d, ocelli; e, maxilla;

f, labium; g, spinneret; h, labial palp.

Immediately after hatching, many caterpillars eat the egg-shell for their first meal; they then settle down to the business of feeding and growing. It should be remembered that it is entirely on growth made whilst in the caterpillar stage that the size of a butterfly depends. In the course of a day or two the necessity arises for fasting, as moulting, an important event, is about to take place. Having spun a slender carpet of silk on a leaf or twig, the caterpillar secures itself thereto, and then awaits the moment when all is ready for the transformation to commence. After a series of twistings from side to side and other contortions, the skin yields along the back near the head, the head is drawn away from its old covering and thrust through the slit in the back, the old skin then peels downwards whilst the caterpillar draws itself upwards until it is free. The new skin, together with any hairs or spines with which it may be clothed, is at first very soft. In the course of a short time all is perfected, and the caterpillar is ready to enter upon its second stage of growth. At the end of the second stage the skin-changing operation is again performed, and the whole business is repeated two or more times afterwards. Finally, however, when the caterpillar has shed its skin for the last time, the chrysalis is revealed, but with the future wings seemingly free. These, together with the other organs, are soon fixed down to the body by the shell, which results from a varnish-like ooze which covers all the parts and then hardens.

Generally speaking, newly hatched caterpillars, though of different kinds, are in certain respects somewhat alike, but the special characters of each begin to appear, as a rule, after the first change of skin (ecdysis), and these go on developing with each successive stage (stadium) until the caterpillar is full grown. The form assumed in each stage is termed the instar, therefore a caterpillar just from the egg would be referred to as in the first instar; between the first and second changes of skin, as in the second instar, and so on to the chrysalis, which in the case of a caterpillar that moulted, or changed its skin, four times before attaining full growth, would be the sixth instar, and the butterfly would then be the seventh instar. In practice, however, it is usually the stages of the caterpillar alone that are indicated in this way.