SPINNING CATERPILLARS. 307 



have seen caterpillars drop in this way from one to 

 iix feet or more ; and by means of their cable, which 

 they are careful not to break, they climb back with 

 great expedition to their former place* 



The structure of their legs is well adapted for 

 climbing up their singular rope— the six fore-legs 

 being furnished with a curved claw ; while the pro- 

 legs (as they have been termed) are no less fitted 

 for holding them firm to the branch when they have 

 re-gained it, being constructed on the principle of 

 forming a vacuum, like the leather-sucker with 

 which boys lift and drag stones. The foot of the 

 common fly has a similar sucker, by which it is 

 enabled to walk on glass, and otherwise support it- 

 self against gravity. The different forms of the leg 

 and pro-leg of a spinning caterpillar are represented 

 in the figure. 



Leg and Pro-leg of a Caterpillar, greatly magnified. 

 In order to understand the nature of the appa- 

 ratus by which a caterpillar spins its silk, it is to be 

 recollected that its whole interior structure differs 

 from that of warm-blooded animals. It has, pro- 

 perly speaking, no heart, though a long tubular 

 dorsal vessel, which runs along the back, and pul- 

 sates from twenty to one hundred times per minute, 

 has been called so by Malpighi and others: but 

 * See Insect Transformations, p. 186. 



