THE LARVA. 27 



growth are those of the flesh flies, which become from 

 one hundred and forty to two hundred times heavier in 

 twenty-four hours, and hence their great efficiency in the 

 removal of dead and decaying animal matter. 



The age of the larvse refers to the time that the insect 

 spends in this state, which often includes the longest 

 period of its life. We usually find that those larvre that 

 live on dead animals,, on fungi, and on refuse material, 

 spend the shortest time in this state, and those that live in 

 the ground, on the roots of grasses and trees, and on 

 wood, the longest. The former assume the pupa state in 

 a few days or weeks, while the latter require many 

 months, or even years, before they pass through the 

 stages of the larva. 



These difi'erences evidently depend on their kinds of 

 food and the purposes in the economy of nature they were 

 intended to fill. 



8. Preparation for assuming the pupa state. 



Having attained their full size and laid up all the food 

 for the construction of their parts, the larvae prepare 

 themselves for assuming the pupa state, by emptying the 

 intestinal canal of not only its entire contents, but even of 

 its lining membrane. 



Their methods of disposal of themselves in preparation, 

 for this change might group them as follows : — 



1. Those that remaiu in the substance they fed on. 



2. Those that seek a hiding- place or make a flocculent covering 



around themselves. 



3. Those that suspend them- ( Perpendicularly by the tail 



selves by a silken cord. | ^the middfe ^^ "" '' ^"^ 



4. Those that enclose them- ( Formed chiefly or entirelv of silk. 



selves in cases or cocoons. ( Formed chiefly of other substances. 



