EGGS AND CATERPILLARS 13 



the body of the caterpillar, which has no lungs and 

 needs very little air to keep it healthy. 



The skin of caterpillars is chitinous and is harder 

 and stiifer on the segments than between them. It is 

 called an exoskeleton, or outside skeleton. It is cast 

 several times, most caterpillars molting four times, a 

 few less often, and some oftener. Ten molts are the 

 largest number so far observed in any species. Before 

 molting, a caterpillar stays quiet, with its anal props 

 firmly grasping its support, and does not eat for a day 

 or two. Its head-cover, or mask, is pushed forward 

 by the larger one which is rapidly forming beneath it, 

 until it looks almost like a nose-bag fastened on the 

 new head. When ready to cast the old skin the cater- 

 pillar begins to squirm, contracting and expanding its 

 body in a queer way; then the skin bursts near the 

 head and the caterpillar crawls out of it, pushing the 

 old skin back by means of this muscular contraction 

 and expansion. A fluid has been secreted between the 

 old skin and the new, which enables the caterpillar to 

 push the old skin off without harming the new skin. 

 Not only is the skin cast in molting, but the membrane 

 lining the intestines and spiracles is cast at the same 

 time. The mask sometimes is pushed off when the 

 caterpillar is partly out of the old skin, sometimes is 

 retained until the crawler has rested after the effort of 

 molting, and once in a while declines to be removed 

 by any efforts of the caterpillar ; we have had to take 

 it off in order to enable the larva to eat. The new 

 head is very large after each molt, as after hatching, 

 but the body soon grows up to it, and then becomes 

 larger in proportion before the next molt. Many 



