FLIGHT OF INSECTS. 



185 



on the cylinder. The following are the results obtained as 

 the expressions of the number of movements of the wing per 

 second in each species : — 



Common fly . . • • .330 



Drone-fly 240 



Bee 190 



Wasp 110 



Humming-bird moth (Macroglossa) . 72 



Dragon-fly 28 



Butterfly (Pontia Rapse) ... 9 



Synchronism of the actiori of the two wings. — By holding the 

 insect in a suitable position we can make both wings rub on 

 the cylinder at the same time. It is then seen, on the 

 tracing, that the two wings act simultaneously, and that both 

 perform the same number of movements. Independently of 

 this, we may easily convince ourselves that there must neces- 

 sarily be a similar motion in both wings. 



If we move one of the wings of an insect recently killed, 

 we shall find that a similar movement is given, in a certain 

 degree, to the other corresponding wing ; if we extend one 

 wing laterally, the other is also extended, if we raise one up, 

 the other rises. The wasp is well suited for this experiment. 



Still, in captive flight, certain insects can perform great 

 movements with one of their wings, while the other only exe- 

 cutes slight vibrations. The dung-fly, for instance, usually 

 affects this kind of alternate flight ; when it is held with the 

 forceps, its two wings rarely move together. The sudden- 

 ness and the unforeseen condition of these alternations, and 

 the violent deviations which they give to the axis of the body, 

 have prevented us from taking the simultaneous tracings of 

 the movement of its two wings, and from ascertaining whether 

 the synchronism continues under these conditions, in spite of 

 the unequal amplitude of the movements. 



The preceding figures show the regular periodicity of the 

 movements of insect flight, but they also prove that the 

 graphic method cannot represent the whole course of the wing, 

 for this organ can only be tangential to a certain portion of 

 the surface of the cylinder. Whatever may be the movements 



