BRANCH ARTHROPOD/1; CLASS 1NSECTA : THE INSECTS 181 



not be so miscalled. These last belong to the branch 

 Arthropoda but to other classes than the class Insecta. 

 While insects are found living under most diverse condi- 

 tions on land, that is, on the ground, in the leaves, fruits 

 and stems of plants, in the trunks of trees or in dead 

 wood, in the soil, in decaying animal or plant matter, as 

 parasites on or in other animals, and in all fresh-water 

 ponds and streams, they do not live in ocean water. A 

 few species live habitually on the surface of the ocean, and 

 a few other forms are found habitually on the water- 

 drenched rocks and seaweeds between tide lines. The 

 varied habits of insects, their economic relations with 

 man. the beauty and grace of many of them, and the 

 readiness with which they may be collected, reared and 

 studied, renders them unusually fit animals for the special 

 attention of beginning students of zoology. 



Body form and structure. The segments composing 

 the body of an insect 

 are grouped to form 

 three body-regions, the 

 head, thorax, and abdo- 

 men. The head of an 

 adult insect appears to 

 be a single segment or 

 body-ring, but in reality 

 it is composed of several 

 segments, probably 

 seven, completely fused. 

 The head bears the eyes, 

 antennae and the mouth- 

 parts. The thorax is 

 made up of three seg- 

 ments, each segment 

 bearing a pair of legs. 

 From the dorsal side of the hinder two thoracic segments 



FIG. 45. A wingless insect; the American 

 spring-tail. Lepidocyrtus americanus, 

 common in dwelling-houses. The short 

 line at the right indicates the natural 

 size. (From Marlatt.) 



