138 



ANIMAL LOCOMOTION. 



upward, downward, forward, or backward direction, but also 

 at various intermediate degrees of obliquity. All wings 

 obtain their leverage by presenting oblique surfaces to the 

 air, the degree of obliquity gradually increasing in a direction 

 from behind forwards and downwards during extension and 

 the down stroke, and gradually decreasing in an opposite 

 direction during flexion and the up stroke. 



In the insect the oblique surfaces are due to the conforma- 

 tion of the shoulder-joint, this being furnished with a system 

 of check-ligaments, and with horny prominences or stops, set, 



Fig. 63.— The Lapwing-, or Green Plover ( Vanellus cristatus, Meyer), with one 

 wing (c 6, d' e' f) fully extended, and forming a long lever; the other {d e /, 

 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 t^j9w«rcZs 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 

 (rf, e,./) 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 pre- 

 sent 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 ef c b) is so fully flexed as in the figure. As a con- 

 sequence, the under surface of the wing is, as a rule, inchned upwards and for- 

 wards, even in flexion, so that it acts as a kite in extension and flexion, and 

 during the up and down strokes. — Original. 



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 



