162 



ANIMAL LOCOMOTION. 



wing during the down stroke is well seen in the dragon-fly, 

 represented at fig. 86, p. 161. 



Here the arrows rs indicate the range of the v/ing. At 

 the beginning of the down stroke the upper or dorsal sur- 

 face of the wing (id f) is inclined slightly upwards and for- 

 wards. As the wing descends the posterior margin (if) 

 twists and rotates round the anterior margin (i d), and greatly 

 increases the angle of inclination as seen at i j, g h. This rota- 

 tion of the posterior margin (i j) round the anterior margin 

 (g K) has the effect of causing the different portions of the under 

 surface of the wing to assume various angles of inclination 

 with the horizon, the wing attacking the air like a boy's kite. 

 The angles are greatest towards the root of the wing and least 

 towards the tip. They accommodate themselves to the speed 

 at which the different parts of the wing travel — a small 

 angle with a high speed giving the same amount of buoying 

 power as a larger angle with a diminished speed. The screw- 

 ing of the under surface of the wing (particularly the posterior 

 margin) in a downward direction during the down stroke is 

 necessary to insure the necessary upward recoil; the wing 

 being made to swing downwards and forwards pendulum 

 fashion, for the purpose of elevating the body, which it does by 

 acting upon the air as a long lever, and after the manner of a 

 kite. During the down stroke the wing is active, the air passive. 

 In other words, the wing is depressed by a purely vital act. 



The down • stroke is readily explained, and its results 

 upon the body obvious. The real difficulty begins with 

 the up or return stroke. If the wing was simply to travel in 

 an upward and backward direction from c to a of fig. 84, 

 p. 160, it is evident that it would experience much resistance 

 from the superimposed air, and thus the advantages secured 

 by the descent of the wing would be lost. What really 

 happens is this. The wing does not travel upwards and 

 backwards in the direction cba of fig. 84 (the body, be it 

 remembered, is advancing) but upwards and forwards in the 

 direction c d e f g. This is brought about in the following 

 manner. The wing is at right angles to the horizon (x x') at 

 c. It is therefore caught by the air at the point (2) because 

 of the more or less horizontal travel of the bod}'- ; the elastic 



