201' ANIMAL MECHANISM. 



wing tends to bring to Lear upon it. The artificial insect, 

 when once set in motion, is sometimes before, and at others 

 behind the horizontal force developed by the wing : on this 

 account the rib of the wing is forced to bend, because the 

 mass to be moved does not obey instantaneously the resulting 

 horizontal force which the wing derives from the resistance of 

 the air. The same phenomenon must take place in the flight 

 of a real insect. 



5. Plane of oscillation of an insect's iriiiy. The apparatus 

 which has just been described does not yet give a perfect idea 

 of the mechanism of insect flight. We have been compelled, 

 for the sake of explaining the movements of the wing more 

 easily, to suppose that its oscillation is made from above 

 downwards ; that is to say, from the back of the insect towards 

 its lower surface, Avhen lying horizontally in the air. 



But we need only observe the flight of certain insects, the 

 common fly, for instance, and most of the other diptera, 

 to see that the plane in which the wings move is not verti- 

 cal, but, on the contrary, very nearly horizontal. This plane 

 directs its upper surface somewhat forward, and therefore 

 the main-rib of the wing corresponds with this surl'm-o. 

 Consecpaeutly, it is from below upwards, and a little forward 

 that the propulsion of the insect is effected. The greater part 

 of the force exerted by the wing will be employed in sup- 

 porting the insect against the action of its weight ; the rest of 

 this impulse will carry it forward. 



By changing the inclination of the plane of oscillation of its 

 wings, which can be done by moving the abdomen so as to 

 displace the centre of gravity, the insect can, according to its 

 wishes, increase the rapidity of its forward flight, lessen the 

 speed acquired, retrograde, or dart toward the side. 



It is easily to be seen that, when a hymenopterous insect 

 living at full speed, stops upon a flower, this insect directs the 

 plane of the oscillation of its wings backwards with considi-r- 

 al>lo force. 



Nothing is more variable, in fact, than the inclination of 

 the plane in which the wings of different species of insects 

 oscillate. 



The diptera nppoar to us to have this plane of oscillation 



