MECHANISM OF INSECT FLIGHT. 



197 



face of the wing, we see this surface carried backwards, while 

 by blowing on it from beneath, we turn the upper surface 

 forwards. In certain species of insects, according to Felix 

 Plateau, the wing resists the pressure of the air acting from 

 below upwards, more than that exerted in an opposite 

 direction. 



Is it not evident, that in the movements which take place 

 during flight, the resistance of the air will produce upon the 

 plane of the wing the same effects as the currents of air which 

 we have just employed ? The changes in the plane, caused 

 by the resistance of the air under these conditions, are pre- 

 cisely those which are observed in flight. We have seen that 

 the descending wing presents its anterior surface forwards, 

 which is explained by the resistance of the air acting from 

 below upwards ; while the ascending wing turns its upper sur- 

 face backwards, because the resistance of the air acts upon it 

 from above downwards. 



It is, therefore, not necessary to look for special muscular 

 actions to produce changes in the plane of the wing ; these, 

 in their turn, will give us the key to the oblique curvilinear 

 movements which produce the figure of 8 course followed by 

 the insect's wing. 



Let us return to fig. 82 : the wing which descends has at 

 the same time a forward motion ; therefore, the inclination 

 taken by the plane of the wing, under the influence of the 

 resistance of the air, necessarily causes the oblique descent 

 from h' to a. An inclined plane which strikes on the air has 

 a tendency to move in the direction of its own inclination. 



Let us suppose, then, that the wing only rises and falls by 

 its muscular action ; the resistance of the air, by pressing on 

 the plane of the wing, will force the organ to move forward 

 while it is being lowered. But this deviation cannot be 

 effected without the nervure being slightly bent. The force 

 which causes the wing to deviate in a forward direction neces - 

 sarily varies in intensity according to the rapidity with which 

 the organ is depressed. Thus, when the wing towards the 

 end of its descending course moves more slowly, we shall 

 see the nervure, as it is bent with less force, bring the wing 

 backwards in a curvilinear direction. Thus we explain 



