232 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



salient border of the opposite wing is rubbed in such a manner as to 

 sound the drum and produce the noise familiar to every one. It will be 

 understood that at present we eliminate all these sounds, the origin of 

 which is evidently different from that of the vibrations of tlie wings, 

 and confine ourselves to the very great number of cases in which the 

 buzzing of the insect is manifestly produced by the rapid strokes of 

 these organs. 



Chabrier and Lacordaire report that a portion of the wings of most 

 insects can be destroyed without a cessation of the sound. " lu portions, 

 as pieces of these organs are cut off, the sound becomes sharper, and sen- 

 sibly more feeble, especially when we leave only a stump remaining. If 

 this last be removed, which can only be done by a considerable laceration 

 of the muscles which attach it to the thorax, the buzzing ceases entirely." 

 " If," concludes the author, " the buzzing were entirely due to the wings, 

 we could not cut off with impunity three-quarters of these organs." ' This 

 objection confirms the hypothesis which it apparently was intended to 

 disprove. In effect, since the sound is elevated in the same ratio in 

 which the vibrating wing is diminished in length, is not this ijhenomenon 

 entirely analogous to that which we observe when we have shortened 

 a vibrating rod ? The modification produced in the sound being the 

 same in both cases, should not the mechanism of its production be iden- 

 tical"? At least the facts cannot be regarded otherwise, if the vibra- 

 tions of the wings are really the cause of the buzzing. 



The authors whom we have j ust cited have indicated an entirely different 

 cause for the acoustic phenomena. They have attributed them to the 

 air which enters the tracheae, and which, passing rapidly outward, puts 

 in vibration the little scaly organs which surround the base of the stig- 

 mati.* And they cite, in contirmation of their views, this fact — that the 

 buzzing also ceases if the surface of the body of the animal is covered 

 with gum so that the excess of air to the respiratory canals is prevented. 

 The lips of the stigmati act as the lips of the glottis do in superior animals, 

 and the buzzing of the insect, therefore, becomes a true voice. Whatever 

 may be the fate of this explanation, the result which we desire remains 

 the same. We see, in effect, that in the motions of the wing there is, 

 as it seems, only a single active period — that in which it is lowered, 

 brought down ; the elevation takes place in consequence of the ehis- 

 ticity of the pieces of the thorax strongly stretched by the contraction 

 of the muscles which pull down the wing. At the same time that this 

 tension is produced, the volume of the thorax is amplified and the incur- 

 sion of air is the immediate result of this increase of capacity. The air 

 must then enter and leave the trachete at each stroke, and the vibra- 

 tions it produces (whether proceeding from the wing membrane or from 

 the stigmati) correspond exactly to the movements of the wing. 



The acoustic phenomenon, if it is not the consequence of the vibra- 

 tion of the wings, is at least synchronous with it. It can inform us, 

 therefore, in all cases of the frequency of the strokes. When the buz- 

 zing of an insect, flying with a uniform rapidity, is observed, we find that 

 the pitch perceptibly does not remain the same ; when the insect ap- 

 proaches the ear the pitch is elevated, and is lowered when the insect 

 flies from us. Something analogous takes place when a tuning-fork in 

 vibration is moved rapidly to and from the ear ; the sound becomes 

 elevated and then low^ered, and the difference may attain a quarter or 

 even half a tone. It is necessary, therefore, to take care that the in- 

 sect experimented on should alv/ays be at the same distance from the 



* stigmati : the spiracles or openings iu the external integument of insects, through 

 "which respiration is carried on. 



