In flexion, as a rule, the under surface of the wing (fig. 84 _a_) is arranged in the same plane with the body, both being in a line with or making a slight angle with the horizon (_x x_).[80] 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 _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 concave or biting surface directed forwards and towards the earth. The same phenomena are indicated at _a b 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 effect of diminishing the angles made by the under surface of the wing with the horizon. Compare _b c d_ of fig. 85 with the same letters of fig. 84. 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. [80] It happens occasionally in insects that the posterior margin of the wing is on a higher level than the anterior one towards the termination of the up stroke. In such cases the posterior margin is suddenly rotated in a downward and forward direction at the 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 downwards, never rises above the anterior one, either during the up or down stroke. [Illustration: FIG. 86.] The downward screwing of the posterior margin of the wing during the down stroke is well seen in the dragon-fly, represented at fig. 86, p. 161.