Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . is merely to serve as props tosupport the body while the insect is walking,and to prevent its hinder part from trailing onthe ground. They are frequently terminated bysingle or double hooks; and also by a marginalcoronet of recurved spines. These hooks, orspines, enable the insect to cling firmly tosmooth surfaces; and also to grasp the mostslender twig, which could not have been laidhold of by legs of the usual construction. The speed with which these larvae can ad-vance is regulated by ma
Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . is merely to serve as props tosupport the body while the insect is walking,and to prevent its hinder part from trailing onthe ground. They are frequently terminated bysingle or double hooks; and also by a marginalcoronet of recurved spines. These hooks, orspines, enable the insect to cling firmly tosmooth surfaces; and also to grasp the mostslender twig, which could not have been laidhold of by legs of the usual construction. The speed with which these larvae can ad-vance is regulated by many circumstances inde-pendently of the mere possession of legs: forsome caterpillars move slowly, while others canrun very nimbly. The following is the order inwhich the legs are usually moved: namely, theanterior and the posterior leg on the same sideare advanced at the same moment, together with TERRESTRIAL LARVyE. 315 the intermediate one on the other side; and thistakes place alternately on both sides. There is one tribe of caterpillars called Sur-veyors, or Geometers, (Fig. 148*, a) which walk. by first fixing the fore feet, and then doublingthe body into a vertical arch ; this action bringsup the hind part of the caterpillar, which is fur-nished with prolegs, close to the head. Thehind extremity being then fixed by means ofthe prolegs situated at that part, the body isagain extended into a straight line ; and thisprocess being repeated, the caterpillar advancesby a succession of paces, as if it were measuringthe distance, by converting its body into a pairof compasses. At the same time that they em-ploy this process, they further provide for theirsecurity by spinning a thread, which they fastento different points of the ground as they goalong. * Many other species of caterpillar practise the * The great force exerted by the muscles of many caterpil-lars is exemplified by their often fixing themselves to an object,and extending the body to a distance, as if it were a rigid
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