134 NATURAL HISTORY OF AQUATIC INSECTS CH. 



upon the floating pupa-case, as if to dry its wings, 

 and then flies away. The whole business of extrica- 

 tion occupies less than a minute. 



During the later larval stages, a number of new 

 organs are developed, which only become useful in 

 the winged Insect. Three pairs of long and slender 

 legs ultimately replace the very different larval feet ; 

 a pair of gauzy wings are added, and the head 

 becomes completely reconstructed. The powerful 

 mandibles and spinning appliances of the larva are 

 useless to the fly ; on the other hand, the fly will 

 want far more elaborate sense-organs than those of 

 the larva. The larva is an animal of very simple 

 mode of life, feeding upon dead vegetable matter at 

 the bottom of dark and slow streams ; the fly is a 

 nimble, aerial Insect, requiring keen senses to escape 

 danger and find a mate. The head of the larva is 

 accordingly both simple and small ; that of the fly 

 not only quite different in shape and far more complex 

 in structure, but absolutely larger. As a mere matter 

 of dimensions the head of the fly could not be con- 

 tained in the larval head. 



Provision is made, not only in Chironomus, but in 

 ver}' many Insects, for the gradual development of 

 new parts, differing materially from the old ones. An 

 Insect elaborates by secretion from its epidermis a 

 continuous integument or cuticle, which at length 

 becomes hard and firm. Whenever the cuticle proves 

 too small for the growing body within, the epidermis 

 is retracted, and develops a new cuticle within the 

 old one. The old cuticle ultimately bursts open, and 

 the Insect emerges, clothed in its new and very flexible 



