FLIGHT OF INSECTS. 



181 



3. How is the motive force which sustains and transports 

 the body of the animal developed ? 



1. Frequency of the movements of the wing of insects. — The 

 frequency of the movements of the wing varies according to 

 species. The ear perceives an acute sound during the flight 

 of mosquitos and certain flies ; there is a graver sound during 

 the flight of the bee and the drone fly ; still deeper in the 

 macroglosssB and the sphingidae. As to the other lepidoptera, 

 they have, in general, a silent flight on account of the few 

 strokes which they give with their wings. 



Many naturalists have endeavoured to determine the fre- 

 quency of the strokes of the wing by the musical note pro- 

 duced by the animal as it flies. But in order that this deter- 

 mination should be thoroughly reliable, it must be clearly 

 established that the sound produced by the wing depends 

 exclusively on the frequency of its movements, in the same 

 manner as the sound of a tuning-fork results from the fre- 

 quency of its vibrations. But opinions difier on this subject ; 

 certain writers have thought that during flight there is a 

 movement of the air through the spiracles of insects, and that the 

 sound which is heard depends on these alternate movements. 



Without giving our adherence to this opinion, which 

 seems to be contradicted by many facts, we think that the 

 acoustic method is insufiicient to furnish an estimate of the 

 frequency with which the wing moves. The reason which 

 would induce us not to employ this method, is that the 

 musical note produced by the flying insect is varied by other 

 influences besides the changes in the strokes of the wing. 



When we observe the buzzing of an insect flying with a 

 uniform rapidity, we perceive that the tone does not continue 

 constantly the same. As the insect approaches the ear, the 

 tone rises ; it sinks as it goes farther from us. Something of 

 an analogOTjs kind happens when we cause a vibrating 

 tuning-fork to pass before the ear ; the note at first becomes 

 more shrill and then more grave, and the difference may 

 attain to a quarter or even to half a tone. We must, there- 

 fore, take care that the insect on which we experiment should 

 be always at the same distance from the observer. This dis- 

 turbing phenomenon, however, presents no real difficulty of 



