*282 THE MECHANICAL FUNCTIONS. 



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 provide 

 for their security by spinning a thread, which they 

 fasten to different points of the ground as they go 

 along. The great force exerted by the muscles of 

 many caterpillars is exemplitied by their often 

 fixing themselves to the slender branch of a tree, 

 and extending the body, as if it were a rigid cylin- 

 der, at an acute angle, so as exactly to resemble 

 one of the twigs from the same branch, and thus 

 escape the observation of their enemies. This atti- 

 tude, which they persist in assuming for hours and 

 days together, is shown in Fig. 148* b. 



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 descending from a 

 height through the air. The caterpillar of the cab- 

 bage butterfly is thus enabled to climb up and 

 down a pane of glass, for which purpose it fixes the 

 threads which it spins in a zig-zag line, forming so 

 many steps of a rope ladder. The material of which 

 these threads are made is a glutinous secretion, 

 which, on being deposited on glass, adheres firmly 

 to it, and very soon acquires consistence and hard- 

 ness by the action of the air. 



Other caterpillars, which feed on trees, and have 

 often occasion to descend from one branch to an- 

 other, send out a rope made with the same material, 

 which they can prolong indefinitely ; and thus 

 either suspend themselves at pleasure in the air, or 



