ANATOMY AND PHYSIOLOGY 



B 



The smaller the wings are, the more rapidly they vibrate. Thus a 

 butterfly (P. rapa) makes 9 strokes per second, a dragon fly 28, a sphin- 

 gid moth 72, a bee 190 and a house-fly 330. 



Wing Muscles. The base of a wing projects into the thoracic cavity 

 and serves fdr the insertion of the direct muscles of flight. Regarding the 

 wing as a lever (Fig. 75, A), with the fulcrum at p } it is easy to understand 

 how the contraction of muscle e 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 in- 

 directly 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 ter- 

 gum of the thorax. 



Though up and down move- 

 ments are all that are necessary 

 for the simplest kind of insect 

 flight, the process becomes com- 

 plex in proportion to the effi- 

 ciency of the flight. Thus in 

 dragon flies there are nine 

 muscles to each wing: five de- 

 pressors, three elevators and 

 one adductor. The earlier ac- 

 counts of the mechanics of flight 

 by Marey and others have been 

 recently modified and improved upon by Stellwaag and by Ritter, whose 

 modern methods of investigation have added considerably to our knowl- 

 edge of the subject. They show, particularly, the parts played by the 

 thoracic sclerites during flight. 



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. 



FIG. 75. A, diagram to illustrate the action 

 of the wing muscles of an insect. B, diagram of 

 wing muscles, a, alimentary canal; en, muscle 

 for contracting the thorax, to depress the wings; 

 d, depressor of wing; e, elevator of wing; ex, 

 muscle for expanding the thorax, to elevate the 

 wings; id, indirect depressor; ie, indirect ele- 

 vator; /, leg muscle; p, pivot, or fulcrum; s t 

 sternum; t, tergum; wg, wing. After GRABER. 



