ANATOMY AND PHYSIOLOGY 



FIG. 75- 



B 



with the fulcrum at p, it is easy to understand how the con- 

 traction of muscle c raises the wing and that of muscle d low- 

 ers it. These muscles are shown diagrammatically in Fig. 

 75, B. Besides these, there are certain muscles of flight which 

 act indirectly upon the wings, by altering the form of the 

 thoracic wall. Thus 

 the muscle ie (Fig. 75, 

 B) elevates the wing 

 by pulling the tergum 

 toward the sternum ; 

 and the longitudinal 

 muscle id depresses the 

 wing indirectly by 

 arching the tergum of 

 the thorax. 



Though up and 

 down movements are 

 all that are necessary 

 for the simplest kind 

 of insect flight, the 

 process becomes com- 

 plex in proportion to 

 the efficiency of the 

 flight. Thus in dragon 

 flies there are nine 

 muscles to each wing: 

 five depressors, three 

 elevators and one ad- 

 ductor. 



Abdomen. - The 

 chief functions of the 

 abdomen are respiration and reproduction, to which should be 

 added digestion. The abdomen as a whole has undergone less 

 differentiation than the thorax and presents a simpler and more 

 primitive segmentation. 



Segments. A typical abdominal segment bears a dorsal 

 6 



A, diagram to illustrate the action of the wing 

 muscles of an insect. B, diagram of wing mus- 

 cles, a, alimentary canal; en, muscle for con- 

 tracting the thorax, to depress the wings; d, de- 

 pressor of wing; e, elevator of wing; ex, muscle 

 for expanding the thorax, to elevate the wings; 

 id, indirect depressor; ie, indirect elevator; /, leg 

 muscle; p, pivot, or fulcrum; s, sternum; t, ter- 

 gum; wg, wing. After GRABER. 



