FLIGHT OF INSECTS. 347 



body of the insect in the air. The action in 

 which the muscles exert the greatest force is in 

 striking the air during the descent of the wing . 

 an impulse in the opposite direction being the 

 result of the reaction of the air. The axis of 

 motion of the wings is a line inclined at a small 

 angle to the axis of the body, and directed from 

 before backwards, outwards, and downwards ; 

 and they move in a plane, which is not vertical, 

 but inclined forwards. The angle which the 

 plane of the wing forms with the horizon varies 

 continually in the different positions of the wing, 

 but the general resultant of all these successive 

 impulses is a force directed forwards and 

 upwards ; the first part of this force produces 

 the horizontal progression of the insect, while 

 the second operates in counteracting the force 

 of gravity, and during the advance of the in- 

 sect, either maintains it at the same height, or 

 enables it to ascend. 



When the insect wishes to turn, or to pursue 

 an oblique course, it effects its purpose very 

 easily by striking the air with more force on one 

 side than on the other ; or, by employing certain 

 muscles which bend the body to one side, it 

 shifts the situation of the centre of gravity, so 

 that the reaction of the air on the wings i& 

 exerted in a different direction to what it was 

 before ; and the motion of the body is modified 

 accordingly. 



