I 



CHAPTER 18 



Phylum Artliropnda. 



Insects 



NSECTS are more numerous in species than 

 all other animals combined. Over 850,000 

 species have been described; and, no doubt, 

 hundreds of thousands remain to be dis- 

 covered (Fig. 130). They live in almost 

 every conceivable type of environment: on 

 land and in water, from the Arctic to the 

 tropics; and their structure, habits, and life 

 cycles are correspondingly modified. Never- 

 theless, it is possible to separate this vast 

 assemblage into orders, families, etc., al- 

 though there is no unanimity of opinion 

 with respect to the number that should be 

 recognized and the names that should be 

 applied (Fig. 129). 



The insects are such a large and varied 

 group that many specialties have grown up 

 in the general field of entomology, for ex- 

 ample, medical and economic entomology. 

 Because insects differ so widely in habit, 

 physiology, and morphology and exhibit so 

 many interesting adaptations, only those 

 areas of entomology are included that deal 

 briefly with life cycles, adaptive modifica- 

 tions, types of coloration, and social life. 



The class Insecta are air-breathing arthro- 

 pods with bodies divided into head, thorax, 

 and abdomen. The head bears one pair of 

 antennae, and the thorax three pairs of legs, 

 and usually one or two pairs of wings in the 

 adult stage. 



In many ways the grasshopper is a very 

 favorable species as a type for detailed study. 

 It is abundant and easily secured; it is com- 

 paratively large and hence excellent material 

 for dissection; it is one of the least special- 

 ized of all insects, and, therefore, exhibits 

 better than most other forms the essential 

 features of insects, both externally and in- 

 ternally. The grasshopper also has several 

 conspicuous adaptations, such as leathery 



228 



Figure 129. Facing page, representatives of some 

 orders of insects. The lines suggest possible relation- 

 ships. The figures are not drawn to scale. (Drawn 

 specifically for this text from a diagram prepared by 

 R.L. Fischer, Curator of Insects, Michigan State 

 University. ) 



