PROGRESSION IN OK THROUGH THE AIR. 



161 



When the wing is made to descend, it gradually, in virtue of 

 its simultaneously rotating upon its long and short axes, 

 makes a certain angle with the horizon as represented at J. 

 The angle is increased at the termination of the down stroke 

 as shown at c, so that the wing, particularly its posterior 

 margin, during its descent (A), is screwed or crushed down 

 upon the air with its concave or biting surface directed for- 

 wards and towards the earth. The same phenomena are 

 indicated at 6 c of fig. 85, but in this figure the wing is 

 represented as travelling more decidedly forwards during its 

 descent, and this is characteristic of the down stroke of the 

 insect's wing — the stroke in the insect being delivered in a 

 very oblique and more or less horizontal direction (figs. 64, 

 65, and 66, p. 139 ; fig. 71, p. 144). The forward travel of 

 the wing during its descent has the efl[ect of diminishing the 

 angles made by the under surface of the wing with the hori- 

 zon. Compare bed of fig. 85 with the same letters of fig. 84. 



Pig, 86. 



At fig. 88 (p. 166) the angles for a similar reason are still 

 further diminished. This figure (88) gives a very accurate 

 idea of the kite-like action of the wing both during its 

 descent and ascent. 



The downward screwing of the posterior margin of the 



and forward direction at the beginning of the down stroke— the downward and 

 forward rotation securing additional elevating power for the wing. The pos- 

 terior margin of the wing in bats and birds, unless they are flying downwards, 

 never rises above the anterior one, either during the up or down stroke. 



