ANATOMY AND PHYSIOLOGY 



49 



of the cotyla medianward, and as the wing swings farther back the middle 

 system of veins (ni) is pushed outward and anteriorly. This motion, 

 combined with the backward movement of the wing as a whole, produces 

 the folding of the distal end of the wing. There are no traces of muscles 

 or elastic ligaments in the wing which could aid in the folding. 



Mechanics of Flight. The mechanism of insect flight is much less 

 complex than one might anticipate. Indeed, owing to the structure of 

 the wing itself, simple up and down movements are sufficient for the 

 simplest kind of flight. During oscillation, the plane of the wing changes, 

 as may be demonstrated by 

 holding a detached wing by 

 its base and blowing at right 

 angles to its surface; the 

 membrane of the wing then 

 yields to the pressure of the 

 air while the rigid anterior 

 margin does not, to any 

 great extent. Similarly, as 

 the wing moves downward 

 the membrane is inclined 

 upward by the resistance of 

 the air, and as the wing c '' 

 moVes upward the mem- 

 brane bends downward. 

 Therefore, by becoming de- 

 flected, the wing encounters 



FIG. 72. Wing of Lcpthiotarsa dcccmlincata. A, 



spread; B, folded; a, costal joint; all, articular head; 

 an, anterior system of veins; b, transverse vein; c, 

 cotyla; d, joint; m, middle system of veins; /^poste- 

 rior system of veins. After TOWER. 



a certain amount of resist- 

 ance from behind, which is 

 sufficient to propel the insect. 

 The faster the wings vibrate, 



the greater the deflection, the greater the resistance from behind, and 

 the faster the flight of the insect. 



The path traced in the air by a rapidly vibrating wing may be deter- 

 mined by fastening a bit of gold leaf to the tip of the wing and allowing the 

 insect a wasp, for example to vibrate its wings in the sunlight, against 

 a dark background. Under these conditions, the trajectory of the wing 

 appears as a luminous elongate figure 8. During flight, the trajectory 

 consists of a continuous series of these figures, as in Fig. 73. 



Marey, the chief authority on animal locomotion, used chronopho- 

 tography, among other methods, in studying the process of flight, and 

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