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_). 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. [Illustration: FIG. 1. Young caterpillar of Orange-tip highly magnified. (_After Sharp._)]