TERRESTRIAL LAUVJE. 223 



the winding passages which thc}^ have themselves excavated, 

 holding by the jaws, and dragging themselves forwards. 

 These movements are assisted by the resistance aflbrded by 

 short tubercles which project from different parts of the back 

 and under surface of the body; so that these insects advance 

 in the passage by an act similar to that by which a chimney- 

 sweeper, exerting the powerful pressure of his elbows, shoul- 

 ders, and knees, manages to climb up a chimney. 



For the purpose of enabling insects to take stronger hold 

 of the surfaces they pass over, we often observe them fur- 

 nished with spines, or hooks, which are moved by appro- 

 priate muscles, and they occupy different situations on the 

 body. Modifications without end occur with regard to these 

 and other external parts, subservient in various degrees to 

 progressive motion. Every possible gradation is also seen 

 between the short tubercles already mentioned, and the more 

 regularly formed feet or legs. Those which are regarded as 

 spurious legs, or prolegs, as they have been called, occupy 

 an intermediate place between these two extremes. They 

 consist of fleshy and retractile tubercles, and are often very 

 numerous; while the number of the l?nie 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 tlie thorax; and are formed of 

 those portions articulated to each other, corresponding to the 

 three principal joints of the imago. The true legs are gene- 

 rally protected Ijy horny scales; but the coverings of the pro- 

 legs are wholly membranous. The office of these spurious 

 legs is merely to serve as props to support tlic 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 bv Icirs 

 of the usual construction. 



The speed with which these larvae can advance is regu- 

 lated by many circumstances, independently of the mere 



