224 THE MECHANICAL FUNCTIONS. 



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, tosether with the intermediate one on the other 

 side; and this takes place alternately on both sides. 



There is one tribe of caterpillars called Surveyors, 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 ex- 

 tremity 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 employ this process, they farther pro- 

 vide for their security by spinning a thread, v/hich they 

 fasten to different points of the ground as they go along. "" 



Many other species of caterpillar practise the same art of 

 spinning fine silken threads, which especially assist them in 

 their progression over smooth surfaces, and also in descend- 

 ing from a height through the air. The caterpillar of the 

 cabbage butterfly is thus enabled to climb up and down a 

 pane of glass, for which purpose it fixes the threads that it 

 spins in a zig-zag line, forming so many steps of a rope lad- 

 der. The material of which these threads are made is a glu- 



* The great force exerted by the muscles of many caterpillars Is exempli- 

 fied 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* b. 



