INTRODUCTION 3 



attached at or near their bases, are the wings — rigid enough, 

 since they consist largely of firm cuticle, to support in the 

 air the creature that bears them, so that insects are among 

 the few groups of animals which possess the power of true 

 flight ; like the birds they are moulded to the air. There 

 are indeed many insects wingless or incapable of flight, the 

 best known examples being such parasites as lice or fleas. 

 But taking the class as a whole the wings are a dominant 

 feature and flight is a characteristic activity, so that the vast 



Fig. 2. — A, Stonefly {Taeniopteryx pacified) North America, X 5 ; 

 By mandibles ; C, maxilla ; Z), labium ; X 25. From E. J. Newcomer, 

 Journ. Agric. Res. (U.S.D.A.) xiii, 1918. 



majority of insects may be regarded as segmentally built* 

 armoured creatures, able not only with nice precision to 

 walk or run, but to rise into and propel themselves through 

 the air. 



Such strenuous activity calls for a constant supply of 

 energy. In all animals the source of the energy dissipated 

 in motion, in the radiation of heat, or in other channels, 

 must be sought in the highly complex chemical constitution 

 of the living body-substance the food materials combined 

 or in association with which are broken down by oxidation- 



