350 DR PETT1GREW ON THE PHYSIOLOGY OF WINGS. 



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 repre- 

 sented at b. 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 concavity or biting 

 surface directed forwards and towards the earth. The same phenomena are 

 indicated at a b c of fig. 17, p. 349, 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, as shown at Plate XL fig. 4. 

 The forward travel of the wing during its descent has the effect of diminishing the 

 angles made by the under surface of the wing with the horizon. Compare bed 

 of fig. 17 with the same letters of fig. 16. At fig. 15, page 345, the angles for a 

 similar reason are still further diminished, and this latter figure 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 wing during 

 the down stroke is well seen in the dragon-fly at page 361, fig. 38. (In this 

 figure the arrows r s give the range of the wing.) At the beginning of the down 

 stroke (dragon-fly) the upper or dorsal surface of the wing (i df) is inclined 

 downwards and backivards, the under or ventral surface downwards and for- 

 ivards. In other words, the anterior margin (i d) of the pinion is directed 

 slightly upwards and forwards, the posterior margin (/) slightly downwards 

 and backwards. As the wing descends, which it does in a downward and 

 forward direction, the posterior margin (/) is screwed downwards and for- 

 wards until it assumes the position indicated by j ; the anterior margin (i d) 

 inclining more and more upwards and backwards, as shown at g h. This rota- 

 tion of the posterior margin (/) round the anterior margin (g h) 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 portions 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 screwing of the under surface of the wing (particularly the posterior margin) 

 in a downward direction during the down stroke is necessary to insure a sufficient 

 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 



beginning of the down stroke — the downward and forward rotation securing additional elevating 

 power for the wing. The posterior margin of the wing in bats and birds, unless they are flying down- 

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



