XIX. INSECTS 



Insects and Crustaceans Compared. — Both crustaceans and 

 insects belong to a great group of animals which agree in that 

 they have jointed appendages and bodies, and that they possess 

 an exoskeleton. This group is known as. the Art/iropoda. 



Insects differ structurally from crustaceans in having three 

 regions in the body instead of two. The number of legs (three 

 pairs) is definite in the insects ; in the crustaceans the number some- 

 times varies (as in the Entomostraca), but is ahvays more than 

 three pairs. The exoskeleton, composed wholly of chitin in the 

 insects, is usually strengthened with lime in the crustaceans. Both 

 groups have compound eyes, but those of the Crustacea are stalked 

 and movable. The crustaceans receive sensations of touch by 

 means of sensory hairs which protrude from the exoskeleton. The 

 other sense organs do not differ greatly. The most marked differ- 

 ences are physiological. The crustaceans take in oxygen from the 

 water by means of gills, while the insects are air breathers, using 

 for this purpose air tubes called tracheoe. 



The young of both insects and crustaceans usually undergo 

 several changes in form before the adult stage is reached. 

 They are thus said to pass through a metamorphosis. Both 

 insects and crustaceans, because of their exoskeleton, must 

 molt in order to increase in bulk. 



The insects are divided into a number of large groups called 

 orders. The insects found in each order possess certain char- 

 acters in common. We shall now examine several representatives 

 from some of the different orders of insects commonly met with. 



The Order Orthoptera 



The Locust. — The locust or short-horned grasshopper is a 

 type of the class Insecta, which is characterized by possessing a 



227 



