PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 231 



tirely free and placing it before us, the wings being in motion, we 

 perceive two oblique lines, which mark the limits of passage of each of 

 its wings, while their intermediate positions continue to escape the most 

 acute observation. 



It is hardly necessary to give an explanation of this fact. Every time 

 that a body in rapid motion changes its direction to return on its steps 

 and to traverse in a contrary direction the road which it has already 

 traveled, a moment arrives when its motion, before change of direc- 

 tion, becomes absolutely nothing ; this point of cessation, this dead- 

 l)oint, is the extreme limit of oscillation. This is why the impression 

 is produced before the wing has left the limiting position, and at the 

 moment when the impression is about to be eftaced the wing has had 

 time to make a complete oscillation and to return to its point of depart- 

 ure, so that the new impression is confounded with the old one, and the 

 eye experiences a continuous sensation, the result of the fusion of the 

 intermittent sensations. But since the optical method is insufficient to 

 inform us of the frequency of the vibrations of the wings, we must 

 have recourse to another process. I may say, in advance, that the 

 graphic process is the most exact of any we possess, and you shall be 

 the judges of the results it will afford us. However, before broaching 

 this new subject I cannot dispense with saying a few words on an inge- 

 nious method based on the observation of the sound which insects pro- 

 duce during their flight, and which we may designate as the acoustic 

 method. 



Every one has heard the sound which insects produce while flying. 

 If this vibratory buzzing is due to the strokes of the wing, and if they 

 result from its alternate motion back and forth, as the sound of the reed 

 results from the vibration of a metallic lamina, then in ascertaining the 

 pitch of the sound the number of alternate vibrations to which it cor- 

 responds will be known. For this, a tuning-fork, or a piano, is sufficient, 

 with which we can compare the pitch of the sound produced by the 

 insect. This method would be very conclusive if the principle upon 

 which it rests was incontestably established. But on this i^oint there is 

 a difference of opinion among naturalists. First, a number of sounds 

 produced by insects can be definitely excluded, which are certainly not 

 produced by the vibration of the wings. Certain coleoptera, for exam- 

 ple, produce sound by rubbing tlie last upper segments of the abdomen 

 against the elytra.* T. Lacordaire cites, among the insects which emit 

 sound in this manner, the JSfecropliori, the Copri, the exotic Scarabi., and a 

 host of lamellicorn beetles not found in Europe. Almost all the lamelli- 

 corns produce a deep sound by rubbing the peduncle of their mesothoraxf 

 iigainst the prothorax| in which it is inserted. Certain of the CicindelidWj 

 the Oxycheila tristis, the Melasomidcv^ the Cacicus americanus^ rub their 

 thighs or their hinder legs against the border of their elytra, pro- 

 ducing in this way a peculiar noise. Ollivier, in the first volume of his 

 entomology, mentions the fact that the female of a Cape of Good Hope 

 insect, the Moluris striata, calls the male by rubbing a granular j)rotuber- 

 ance upon the lower part of the second segment of the abdomen against 

 extraneous objects. Finally, among the crickets one part of the anterior 

 wings, thinner than the rest, forms a kind of drum or tympanum ; one 

 of the nervures which traverses this drum is armed with denticles, on 

 which, during the alternate motion of the wings one over another, the 



* Elytra: the hard outer wings of beetles. 



t Mesothorax : the second segment of tlie middle or leg-bearing division of the body 

 -of a beetle. 



i Proiltorax : the first segment of the same. 



