FIG. 63.--The Lapwing, or Green Plover (_Vanellus cristatus_, Meyer), with one wing (_c b_, _d´ e´ f´_) fully extended, and forming a long lever; the other (_d e f_, _c b_) being in a flexed condition and forming a short lever. In the extended wing the anterior or thick margin (_d´ e´ f´_) is directed _upwards_ and _forwards_ (_vide_ arrow), the posterior or thin margin (_c_, _b_) _downwards_ and _backwards_. The reverse of this happens during flexion, the anterior or thick margin (_d_, _e_, _f_) being directed _downwards_ and _forwards_ (_vide_ arrow), the posterior or thin margin (_c b_) bearing the rowing-feathers _upwards_ and _backwards_. The wings therefore twist in opposite directions during extension and flexion; and this is a point of the utmost importance in the action of all wings, as it enables the volant animal to rotate the wings on and off the air, and to present at one time (in extension) resisting, kite-like surfaces, and at another (in flexion) knife-like and comparatively non-resisting surfaces. It rarely happens in flight that the wing (_d e f_, _c b_) is so fully flexed as in the figure. As a consequence, the under surface of the wing is, as a rule, inclined upwards and forwards, even in flexion, so that it acts as a kite in extension and flexion, and during the up and down strokes.--_Original._] In the insect the oblique surfaces are due to the conformation of the shoulder-joint, this being furnished with a system of check-ligaments, and with horny prominences or stops, set, as nearly as may be, at right angles to each other. The check-ligaments and horny prominences are so arranged that when the wing is made to vibrate, it is also made to rotate in the direction of its length, in the manner explained. In the bat and bird the oblique surfaces are produced by the spiral configuration of the articular surfaces of the bones of the wing, and by the rotation of the bones of the arm, forearm, and hand, upon their long axes. The reaction of the air also assists in the production of the oblique surfaces. [Illustration: FIG. 64. FIG. 64 shows left wing (_a_, _b_) of wasp in the act of twisting upon itself, the tip of the wing describing a figure-of-8 track (_a_, _c_, _b_). From nature.--_Original._] [Illustration: FIG. 65. FIG. 66. FIGS. 65 and 66 show right wing of blue-bottle fly rotating on its anterior margin, and twisting to form double or figure-of-8 curves (_a b_, _c d_). From nature.--_Original._]