MECHANISM OF INSECT FLIGHT. 207 



placed so that its wings may move in a horizontal plane, the 

 main-rib being uppermost. Thus all the motive force is 

 directed from below upwards, and as soon as the pump begins 

 to act, we see the insect rise vertically. We can easily esti- 

 mate the weight raised by the flapping of the wings, and as 

 we can vary the weight of the insect by altering the position 

 of the counterpoise, we can determine the effort which is 

 developed according to the frequency or the amplitude of the 

 strokes. 



By turning the insect half way round, so that its wings, 

 still oscillating in a horizontal plane, should turn their main- 

 rib downwards, we develop a descending vertical force which 

 may be measured by removing the counterpoise to a greater 

 or less distance, and causing it to be raised by the descent of 

 the insect. 



If we adjust the plane of oscillation of the wings vertically, 

 the insect turns horizontally round its point of support in 

 the same manner as has been previously described and 

 represented in fig. 8 7. 



Lastly, if we give to the plane of oscillation of the wings, 

 the oblique position which it presents in the greater number 

 of insects ; that is to say, so that the main-rib turns at once 

 upwards and slightly forward, we see the insect rise against 

 its own weight, and turn at the same time round the vertical 

 axis ; in a word, the apparatus represents the double effect 

 which is observed in a flying insect, which obtains from the 

 stroke of its wings, both the force which sustains it in the 

 air, and that which directs its course in space. 



The first of these forces is by far the more considerable ; 

 thus, when an insect hovers over a flower, and we see it 

 illuminated obliquely by the setting sun, we may satisfy our- 

 selves that the plane of oscillation of its wings is nearly hori- 

 zontal. This inclination must evidently be modified as soon 

 as the insect wishes to dart off rapidly in any direction, but 

 then the eye can scarcely follow it, and detect the change of 

 plane, the existence of which we are compelled to admit by 

 the theory and the experiments already detailed. 



A curious point of study would be the movements prepara- 



