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.