314 THE MECHANICAL FUNCTIONS. 



between these two extremes. They consist of 

 fleshy and retractile tubercles, and are often 

 very numerous ; while the number of the true 

 legs, as they are called, is limited to six. These 

 last are the representatives of the legs of the 

 future perfect insect; for they are attached to 

 the three first segments of the thorax ; and are 

 formed of those portions articulated to each 

 other, corresponding to the three principal joints 

 of the imago. The true legs are generally pro- 

 tected by horny scales ; but the coverings of the 

 prolegs are wholly membranous. The office of 

 these spurious legs is merely to serve as props to 

 support the body while the insect is walking, 

 and to prevent its hinder part from trailing on 

 the ground. They are frequently terminated 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 ad- 

 vance is regulated by many circumstances inde- 

 pendently of the mere possession of legs : for 

 some caterpillars move slowly, while others can 

 run very nimbly. The following is the order in 

 which the legs are usually moved : namely, the 

 anterior and the posterior leg on the same side 

 are advanced at the same moment, together with 



