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 

 very 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 



