The external organs of the trunk and abdomen are the legs, the spiracles, and various occasional appendages. The legs are of two different kinds. The one, to the number of six, attached by pairs to the trunk, are covered with a shiny cartilage, and armed with hooks. These are the true legs. Fig. 94 represents, after Réaumur's "Mémoire sur les Différentes Parties des Chenilles,"[36] the scaly legs of the caterpillar of the Gipsy Moth. The others are membranous, fleshy, generally conical or cylindrical, contractile, and taking, according to the will of the animal, very different forms. Fig. 95 represents, after the same Memoir of Réaumur's, the different forms of the membranous legs of the silkworm caterpillar. This plate gives a sufficiently good idea of the shape of these organs, and of the hooks, circular or semi-circular, with which they are furnished. [36] Tome i., p. 164; Plate III., Figs. 1, 2. [Illustration: Fig. 95.--Membranous legs of the Silkworm (_Bombyx mori_).] In Fig. 96 are represented, after the same author, two membranous legs of a large caterpillar, of which the hooks of the feet are fastened into a branch of a shrub. [Illustration: Fig. 96.--Membranous legs of a large Caterpillar embracing a twig.] Caterpillars have from two to ten false legs, the scaly legs being always six in number. The pro-legs, as the fleshy ones are called, are divided into _hinder_ and _intermediate_. The former are two in number; the intermediate are rarely more than eight in number. In the caterpillars which have the full number of legs--that is to say, sixteen--there are two empty spaces, where the body has no support: the one between the legs and the pro-legs, formed by the fourth and fifth segment; the other, between the intermediate pro-legs and the anal legs, formed by the tenth and eleventh ring. The variations which caterpillars present, so far as the number and situations of their pro-legs are concerned, are the following:-- The greatest number among them have ten pro-legs; others have only eight; others only six--these may be called semi-loopers; others only four, one pair being situated on the last ring, and the other on the ninth, as in the case of looper caterpillars. And, lastly, there are others which have only two pro-legs. The various forms, numbers, and positions of these organs, produce great differences in the mode of locomotion of caterpillars. Those provided with ten or eight membranous legs have in walking only a very slight undulating motion. Their bodies are parallel to the plane which supports them. They can walk very quickly; but their steps are short and quickly repeated. Others, on the contrary, in proportion as the number of their false legs diminish, and the spaces between the legs increase, walk in a more irregular and quaint manner. [Illustration: Fig. 97.--Looper Caterpillar.] [Illustration: Fig. 98.--Caterpillar curved into an arch.] [Illustration: Fig. 99.--Caterpillar at full length.]