1S-1 ANIMAL MECHANISM. 



Influences which modify the frequency of the movements of the 

 U'UKJ. Since we know the influence of resistance to the rapidity 

 dt' the movements of animals, \ve may suppose that the wing 

 which rubs on the cylinder has not its normal rate of motion, 

 arid that its revolutions are less numerous iu proportion as 

 the friction is greater. Experiment lias confirmed this opinion. 

 An insect performing the movements of flight by rubbing its 

 wings rather strongly against the paper gave 240 movements 

 per second ; by diminishing more and more the contacts of 

 the wing with the cylinder, we obtained still gi-eater numbers 

 282, 305, and 321. This last number may perhaps ex- 

 press with sufficient accuracy the rapidity of the wing when 

 moving freely, for the tracing was reduced to a series of 

 scarcely-visible points. On the contrary, as the wing rubbed 

 more strongly, the frequency of its movements was reduced 

 below 240. 



Another modifying cause of the frequency of movement iu 

 the wing is the amplitude of those movements. We must 

 compare this cause with the preceding, for it is natural to 

 admit that great movements meet with greater resistance in 

 the air than smaller ones. 



AYhen \\e hold a fly or a drone by the forceps, we see that 

 the animal executes sometimes strong movements of flight; 

 we then hear a grave sound; but occasionally, when its wing 

 is only slightly agitated, \ve perceive only a very shrill tone. 

 That which the ear reveals to us with regard to the difference 

 in the frequency of the strokes which the insect gives with its 

 wings, is entirely confirmed by the experiments which wo 

 have made graphically. 



Choosing the instants when the insect is at its strongest 

 (light, and also when it gently flutters its wing, we find that 

 the frequency varies within very extensive limits, nearly in 

 llie proportion of one to three the least frequency belonging 

 to the movements of greatest amplitude. 



The different species of insects on which we have experi- 

 mented, presented also very great variations in the rapidity of 

 the movements of their wings. We have endeavoured as far 

 as possible to compare the different species under similar con- 

 ditions, during their swiftest flight, aud with slight friction 



