_c_, _d_, Posterior margin of wing. _i_, Portion of wing composed of one layer of flexible material. _h_, Portion of wing composed of two layers. _g_, Portion of wing composed of three layers. _f_, Portion of wing composed of four layers. _e_, Portion of wing composed of five layers. _x_, Ball-and-socket joint at root of wing.--_Original._] [Illustration: FIG. 125. FIG. 125.--Flexible _valvular wing_ with india-rubber springs attached to its root. _a_, _b_, Anterior margin of wing, tapering and elastic. _c_, _d_, Posterior margin of wing, elastic. _f_, _f_, _f_, Segments which open during the up stroke and close during the down, after the manner of valves. These are very narrow, and open and close instantly. _x_, Universal joint. _m_, Superior elastic band. _n_, Ditto inferior. _o_, Ditto anterior. _p_, _q_, Ditto oblique. _r_, Ring into which the elastic bands are fixed.--_Original._] _How to construct a Wave Wing which shall evade the superimposed Air during the Up Stroke._--To construct a wing which shall elude the air during the up stroke, it is necessary to make it valvular, as shown at fig. 125, p. 241. This wing, as the figure indicates, is composed of _numerous narrow segments_ (_f f f_), so arranged that the air, when the wing is made to vibrate, opens or separates them at the beginning of the up stroke, and closes or brings them together at the beginning of the down stroke. The time and power required for opening and closing the segments is comparatively trifling, owing to their extreme narrowness and extreme lightness. The space, moreover, through which they pass in performing their valvular action is exceedingly small. The wing under observation is flexible and elastic throughout, and resembles in its general features the other wings described.