268 INSECT ARCHITECTURE. 



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 curA^ed claw ; while the pro-legs (as they have been 

 termed) are no less fitted for holding them firm to the 

 branch when they have regained it, being constructed on 

 the princij^le 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 w^hich it is enabled to 

 walk on glass, and otherwise support itself 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 apparatus by 

 wdiich a caterpillar spins its silk, it is to be recollected 

 that its whole interior structure differs from that of waim- 

 blooded animals. It has, properly speaking, no heart, 

 though a long tubular dorsal vessel, which runs along the 

 back, and pulsates from twenty to one hundred times per 

 minute, has been called so by Malpighi and others : but 

 neither Lyonnet nor Cuvier could detect any vessel issuing 

 from it; and consequently the fluid which is analogous to 

 blood has no circulation. It differs also from the higher 

 orders of animals in having no brain, the nerves running 

 along the body being only united by little knobs, called 

 ganglions. Another circumstance is, that it has no lungs, 

 and does not breathe by the mouth, but by air-holes, or 



