322 



ANIMALS 



and the water near them is kept in constant motion by little plates, 

 or " paddles." The motion of the water is produced by appendages 

 to which the plates are attached. 



As the crayfish increases in size, it must have a larger "house;" so 

 it sheds its outer covering from time to time, and goes into hiding 

 until its new "shell" has grown. 



339. Insects. Insects and spiders have the same 

 general structure as the crayfish, but they also differ from 

 it in many ways. The word insect means "cut into 

 parts/ 7 and indicates that these creatures are made up 

 of joints. True insects have a tough, horny covering, as 

 the crayfish has, but the head and the thorax are separate 

 instead of united. The legs are attached to the thorax. 

 When there are wings, as in flying insects, these are also 

 attached to the thorax. Respiration takes place in tube- 

 shaped air passages in the different segments of the body, 

 and not in gills. The muscles of insects are very highly 

 developed; we see the result in the jumping of the grass- 

 hopper and the flea. 



One of the most fascinat- 

 ing things about insects is 

 the complete change, or 

 metamorphosis (pronounced 

 me t-a-mor '-phos-is) , which 

 they undergo as they de- 

 velop. Thus the eggs of the 

 fly (Fig. 263) produce white, 

 crawling creatures (mag- 

 gots). We call the " worm-like" young of insects larvae 

 (singular, larva). After about a week of rapid feeding, 

 the larva of the housefly grows a "shell" about itself, in 



FIG. 263. 



Pupa 



Stages in the Development of 

 the Housefly. 



