ANIMAL LIFE 355 



identity through life, while the larvae simply represent 

 a stage in the development of insects. 



Insects. All insects have six legs in their mature 

 stage. This feature distinguishes them from crustaceans, 

 which always have more than six legs, usually ten, the 

 spiders, which have eight legs, and from the centipedes, 

 which have many legs. Insects have bodies which are 



FIG. 317. Stages in the Development of the Fly. 



divided into three parts: the head, the thorax, and the 

 abdomen. They breathe air directly by means of air 

 tubes or trachea, which permeate all parts of the body 

 cavity. 



Insects like the common house fly pass through four 

 different periods in their development. The female may 

 lay from one hundred to two hundred eggs. From these 

 eggs the maggots or larvae hatch (Figure 317). After a 

 few days of feeding these maggots go into the pupal 

 stage. In about one more week the adult fly emerges 



