CHAPTER XLV 



CLASS INSECTA 



The fourth class of Arthropoda is Insecta (in sek' ta; L., insectum, 

 having been cut into). The number of described species of insects is 

 enormous; some of the more recent figures place it between 600,000 and 

 700,000, about 75 per cent of all known living animals. It is also certain 

 that a very large number have not yet been described. In the number of 

 known species the class far surpasses all other animal groups combined, 

 and the number of individuals is correspondingly large. Insects are 

 represented everywhere on the land surface of the earth, except at the 

 poles and at the glaciated summits of the highest mountains. They 

 are also numerous in fresh water but are almost entirely absent from the 



Head- 

 Anfznnae\ 



Prof h or ax 



Fig. 180. — A locust, Schistocerca americana Drury, which may serve as a typical insect. 

 (From Lutz, " Fieldbook of Insects," by permission.) Natural size. 



oceans, though one type of true bug is known which occurs on the sur- 

 face of the sea even at a considerable distance from land. The largest 

 insects are certain beetles, the bodies of which reach a length of more 

 than 6 inches, and certain moths, the wing spread of which may be as 

 great as 10 inches. On the other hand, the most minute insects known 

 are no longer than 0.01 inch. 



312. External Characteristics. — Insects agree in having three divisions 

 of the body — head, thorax, and abdomen (Figs. 180 and 181). The 

 metameres represented in the head are so fused as to make it difficult to 

 determine the exact number, but the full number is considered to be six. 

 The thorax contains three, termed in order, prothorax, mesothorax, and 

 metathorax. The first is freely movable, but the two others are fused. 

 There is much variation in regard to the number of metameres present 

 in the abdomen, where the posterior ones are variously modified, but the 

 full number is considered to be 11. 



The head bears a pair of antennae, several mouth parts, and a pair 

 of compound eyes (Figs. 181 and 182). The antennae usually consist of 



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