INSECT AND CRUSTACEANS 



237 



These last-named ones are deep sea crabs and, in some parts of the 

 world, grow to an enormous size. 



Insects and Crustaceans. We have already discussed the 

 characteristics of all insects and the distinguishing features of 

 certain orders of 



insects in Unit III. 



- 



The bodies of all 

 insects are divided 

 into three distinct 

 regions: head, 

 thorax, and ab- 

 domen. Insects 

 have three pairs 

 of legs, breathe 

 through tracheae, 

 usually have two 

 pairs of wings, and 

 undergo a com- 

 plete or incom- 

 plete metamor- 

 phosis. They are 

 found everywhere 

 that life can exist. 

 Insects differ 



structurally from crustaceans in having three regions in the body 

 instead of two. The number of legs is always definite in the in- 

 sects; in the crustaceans the number sometimes varies, but is 

 always more than three pairs. The exoskeleton is composed 

 wholly of chitin x in the insects, but it is sometimes strengthened 

 with lime in the crustaceans. Both groups have compound eyes, 

 but those of the Crustacea are stalked and movable. The other 

 sense organs do not differ greatly. The most marked differences 

 are physiological. The crustaceans take oxygen from the water 

 by means of gills, while the insects are air-breathers, using for this 

 purpose air tubes called tracheae. Both insects and crustaceans, 

 because of their exoskeleton, must molt in order to grow. 



1 Chitin (ki'tin) : a horny substance forming the outer covering of insects. 



fall grown larvcr 



Life history of a moth. Why is the moth classified as an insect ? 

 How does it differ from a crustacean? 



