FIG. 109.—Cross-section through tarsus of a locust: _ch_, cuticula of upper side,—_ch′_, _ch″_, _ch‴_, of sole; _ch_, tubulated layer; _ch″_, lamellate layer; _ch‴_, inner projections of _ch″_. Other lettering as in Fig. 101. See p. 113.—After Dewitz. ] In the saw-flies (Uroceridæ and Tenthredinidæ) and other insects, there are small membranous oval cushions (_arolia_, Figs. 109 and 131) beneath each or nearly each tarsal joint. The triunguline larvæ of the Meloidæ are so called from apparently having three ungues, but in reality there is only a single claw, with a claw-like bristle on each side. =Why do insects have but six legs?=—Embryology shows that the ancestors of insects were polypodous, and the question arises to what cause is due the process of elimination of legs in the ancestors of existing insects, so that at present there are no functional legs on the abdomen, these being invariably restricted (except in caterpillars) to the thorax, and the number never being more than six. It is evident that the number of six legs was fixed by heredity in the Thysanura, before the appearance of winged insects. We had thought that this restriction of legs to the thorax was in part due to the fact that this is the centre of gravity, and also because abdominal legs are not necessary in locomotion, since the fore legs are used in dragging the insect forwards, while the two hinder pairs support and push the body on. Synchronously with this elimination by disuse of the abdominal legs, the body became shortened, and subdivided into three regions. On the other hand, as in caterpillars, with their long bodies, the abdominal legs of the embryo persist; or if it be granted that the prop-legs are secondary structures, then they were developed in larval life to prop up and move the abdominal region. The constancy of the number of six legs is explained by Dahl as being in relation to their function as climbing organs. One leg, he says, will almost always be perpendicular to the plane when the animal is moving up a vertical surface; and, on the other hand, we know that three is the smallest number with which stable equilibrium is possible; an insect must therefore have twice this number, and the great numerical superiority of the class may be associated with this mechanical advantage. (This numerical superiority of insects, however, seems to us to be rather due to the acquisition of wings, as we have already stated on pages 2 and 120.) =Loss of limbs by disuse.=—Not only are one or both claws of a single pair, or those of all the feet atrophied by disuse, but this process of reduction may extend to the entire limb.