FLIGHT OF INSECTS. 



193 



The graphic method also furnishes us with the proof of 

 changes in. the plane of the Aving of the insect during the 

 various instants of its revolutions. 



FIG. 70. Tracings of a Whentstone rod tuned to the octave, furnished 

 with the win^r of a wasp, and arranged so as to register especially the 

 upper loop of the 8. 



Fig. 80 shows the tracing furnished by a wing of a hum- 

 ming-bird moth, arranged so as to touch the cylinder with its 

 posterior edge. Ey bringing the insect not too near, we can 

 succeed in producing only intermittent contacts ; these take 

 place at the moment when the wing describes that part of the 

 loops of the 8 whose convexity is tangential to the cylinder. 

 The contacts which occupy the upper half of the figure alter- 

 nate with those occupying the lower half. It is .seen, besides, 

 that it is not the same surface of the wing which produces 

 these two kinds of friction. In fact, it is evident that the 



FIG. 80. Tracings of the movements of the wing of a humming-bird 

 muth (luacroglossa) rubbing on the cylinder by its lower eclga. 



marks of the upper half, each formed of a series of oblique 

 strokes, are produced by the contact of a fringed border, while 

 the contacts of the lower part are produced by another portion 

 (if the wing which presents a region unprovided with fringes, 

 and leaves a whiter trace witli boundaries better defined. 



