TERRESTRIAL LARV^. 281 



the three principal joints of the imago. The true 

 legs are generally protected by horny integuments; 

 but the coverings of the prolegs are wholly mem- 

 branous. The spurious legs are frequently termi- 

 nated by single or double hooks ; and also by a 

 marginal coronet of recurved spines. These hooks, 

 or spines, enable the insect to cling firmly to smooth 

 surfaces ; and also to grasp the most slender twig, 

 which could not have been laid hold of by legs of 

 the usual construction. 



The speed with which these larvae can advance 

 is regulated by many circumstances, independent 

 of the mere possession of legs ; for some caterpillars 

 move but slowly, while others can run very nimbly. 

 The following is the order in which the legs are 

 usually moved ; namely, the anterior and the pos- 

 terior leg on the same side are advanced at the 

 same moment, together with the intermediate one 

 on the other side ; and this takes 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 doubling the 

 body into a vertical arch ; this action brings up the 

 hind part of the caterpillar, which is furnished with 

 prolegs, close to the head. The hind extremity 

 being then fixed by means of the prolegs situated 

 at that part, the body is again extended into a 



