236 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



oidy a poiut as a mark in eacli of its vibrations. I exhibit a number of 

 these, and trust as soon as the return of sprinjj i)ermits us, to pro- 

 cure insects to show you the experiments by which these tracings have 

 been produced. Those which you are now examining have enabled me 

 to determine the frequency of the strokes of the wings of the following 

 insects : 



strokes 

 per second. 



Common fly 330 



Humble-bee 240 



Honey-bee 190 



Wasp 110 



Sphinx moth, {Macroqlossa) 72 



Dragon fly, {Lihelhda) 28 



Cabbage butterfly 9 



Certain authors have estimated this number of vibrations by the 

 acoustic method, but there is a notable discrepancy between the above 

 figures and those which they have deduced from the pitch of the sound 

 that these iusects produce in flyiug. In the case of the common fly, T. 

 Lacordaire has computed the number of the vibrations of its wings at 

 GOO i^er second, that is to say, twice as many as our figures exhibit. 

 Has there not been a misunderstanding here, as is frequently the case, 

 in the use of the word " vibration f Some persons wrongly consider 

 the raising and depressing of the wing as two vibrations, and reserve 

 the term of "simple vibrations" for one or the other of these isolated 

 motions. On the contrary, if we follow the usage most generally 

 adopted, the two motions together, by which the body is again in its 

 original position, should be considered as a single vibration. 



The previous observations which we have made on free flight, and 

 on flight under restraint, somewhat curtail the range which we are 

 tempted to accord to these numbers. The animal, according as it desires 

 to move with a greater or less rapidity, can change at will not only the 

 extent of its wing-strokes, but also, to a certain extent, their frequency. 

 Fatigue may exercise an analogous influence to that of the will ; after very 

 rapid motions, the exhausted animal diminishes the number of its strokes, 

 which sometioies falls to a fourth or a fifth of its normal value. It con- 

 tinues to relax them more and more until a period of repose and repara- 

 tion permits it to resume its usual flight ; nevertheless the examination 

 of these numbers suggests some general considerations. We have 

 reason to think that each of the muscular contractions which determine 

 the drawing down of the wind is the result of a single impulse, [Zuckimg 

 of the Germans,) although in man contraction is due to successive im- 

 pulses which are merged in one another when they are produced more 

 frequently than 30 times in a second. Among insects the limit of fusion 

 of impulses is infinitely more remote, and ends with leaving the wing 

 immovable in a sort of permanent tetanic contraction. It is easy to 

 assure ourselves of this by means of living iusects, or better, by means 

 of the artificial insect which I have constructed. When the impulses 

 become too rapid, their extent diminishes; at this moment they no longer 

 serve for the propulsion of the animal, whose wings appear quite immov- 

 able or merely agitated by a light tremor. Nevertheless, the number of 

 muscular waves which the fibers of insects will admit without inter- 

 mingling, a number which in the fly amounts to 300 per second, forms a 

 physiological fact, very interesting to note. Among other animals the 

 limit is not so remote; among birds fusion is produced after 75 impulses ; 

 among mammals after 30, and among reptilia after only 4. These dif- 



