196 



ANIMAL MECHANISM, 



CHAPTER II. 



MECHANISM OF THE FLIGHT OF INSECTS. 



Causes of the movements of the wings of insects — The muscles only give 

 a motion to and fro, the resistance of the air modifies the course of 

 the wing— Artificial representation of the movements of the insect's 

 wing - Of the propulsive effect of the wings of insects — Construc- 

 tion of an artificial insect which moves horizontally— Change in the 

 plane in flight. 



1. Causes of the movements of the wing. — These exceedingly 

 complicated movements would induce us to suppose that there 

 exists in insects a very complex muscular apparatus, but 

 anatomy does not reveal to us muscles capable of giving rise 

 to all these movements. We scarcely find any but elevating 

 and depressing forces in the muscles which move the wing ; 

 besides this, when we examine more closely the mechanical 

 conditions of the flight of the insect, we see that an upward 

 and downward motion given by the muscles is sufficient to pro- 

 duce all these successive acts, so well co-ordinated with each 

 other ; the resistance of the air effecting all the other move- 

 ments. 



If we take off the wing of an insect (fig. 83), and holding 

 it by the small joint which connects it with the thorax, expose 

 it to a current of air, we see that the plane of the wing is 



Fig 8u. Htructui e of an insect's wing. 



inclined more and more as it is subjected to a more powerful 

 impulse of the wind. The anterior nervure resists, but the 

 membranous portion which is prolonged behind bends on 

 account of its greater pliancy. If we blow upon the upper sur- 



