PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 233 



observer. Tliis perturbatory phenomenon lias been perfectly explained 

 as follows : Tlie vibrations, without doubt, are all produced at^qual in- 

 tervals of time, and, therefore, when the instrument remains at the same 

 distance from the ear, the same time elapses between the several im- 

 pulses on the tymx)anum. But when the fork is made rapidly to ap- 

 proach the ear the impulses are crowded together, and consequently the 

 pitch rises ; when, on the contrary, the fork is rapidly removed from the 

 ear the impulses are wider apart, and the sound deepens. Every one 

 has remarked, in traveling by rail, that if a locomotive whistles while 

 passing in an opposite direction, the sharpness of the sound increases 

 as it approaches, and becomes deep when it has passed and is rapidly 

 becoming more distant. 



II. The movements of the icing of insects. 



To arrive at a complete comprehension of the mechanism of flight of 

 insects, I have said that we should, in the first place, resolve a certain 

 number of i^ractical questions which should serve us as steps to reach a 

 definite conclusion. I could preseDt you immediately with the final 

 result of the experiments by which I have elucidated this subject for 

 myself, and the theory which expresses them, but I prefer to proceed 

 otherwise. I shall enter on an examination of the facts and into the 

 details of the experiments, in order that my hearers may participate 

 more completely in the studies which we pursue together, for I am per- 

 suaded that there is as much profit in knowing how to arrive at a result 

 as in knowing the result itself. 



1. We have begun to study the motions of the wings, and the first 

 question which presents itself is the frequency of these motions. On 

 this point direct observation is of little assistance ; the acoustic method, 

 which consists in determining the frequency of the strokes of the wing 

 by the pitch of the buzzing of the insect is more efficient, but we have 

 seen that even the principle of this method has been contested, and that 

 its application presents difficulties. The graphic method remains to be 

 considered. This method consists in making the wings themselves 

 record the strokes which they execute. When an insect is held in 

 captivity by force which it cannot overcome, after trial it ceases 

 a useless resistance ; it resigns itself and abstains from all efforts to 

 escape, its wings remain immovable, and in this way the observer who 

 hopes to study their motions finds himself disappointed. But there are 

 different methods of awakening the insect to its original activity ; it 

 is sometimes sufiicient to pinch the antenna3 lightly; this irritation of a 

 very sensitive organ succeeds with the Macroglossa. Among the wasps^ 

 the end may be attained by titillating the feet, or by holding them all 

 together with a j)air of forceps, and then releasing them suddenly, except 

 one, by which the animal is held. The captive supposes that it is at liberty, 

 and makes an effort at flight which last about thirty seconds, or long 

 enough to be observed. There is, however, another difiiculty. The captive 

 insect, when willing, cannot fly like an insect at liberty, because the exter- 

 nal conditions are not the same. It exj>eriences a greater resistance in 

 proportion to the traction which it exerts upon the bond which holds it ^ 

 to a free insect the relation is such as a boat held by an obstruction 

 bears to one sailing freely, or as a horse which drags a load to one re- 

 lieved from harness. This resistance modifies its behavior considerably, 

 and obliges us to distinguish between the two difl'erent conditions of 

 free flight and flight in captivity. It is indispensable to establish these 

 distinctions, in order to apijreciate at their true value the results ta 



