316 THE MECHANICAL FUNCTIONS. 



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 

 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 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 hardness by the action 

 of the air. 



Other caterpillars, which feed on trees, and 

 have often occasion to descend from one branch 

 to another, 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 let themselves down to the ground. 

 They continue, while walking, to spin a thread 

 as they advance, so that they can always easily 

 retrace their steps, by gathering up the clue 

 they have left, and reascend to the height from 

 which they had allowed themselves to drop. 



