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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?

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.)