TERRESTRIAL LARVAE. 315 



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 

 tip the hind part of the caterpillar, which is fur- 

 nished 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 straight line ; and this 

 process being repeated, the caterpillar advances 

 by a succession of paces, as if it were measuring 

 the distance, by converting its body into a pair 

 of compasses. At the same time that they em- 

 ploy this process, they further provide for their 

 security by spinning a thread, which they fasten 

 to different points of the ground as they go 

 along. * 



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 

 cylinder: this attitude is shown in Fig. 148* u. 



