PROGRESSION IN OR THROUGH THE AIR. 



139 



the wing, and by the rotation of the bones of the arm, fore- 

 arm, and hand, upon their long axes. The reaction of the 

 air also assists in the production of the oblique surfaces. 



That the wing twists upon itself structurally, not only in 

 the insectj but also in the bat and bird, any one may readily 

 satisfy himself by a careful examination; and that it twists upon 

 itself during its action I have had the most convincing and re- 

 peated proofs (figs. 64, 65, and 66). The twisting in question 



Fig. 64. 



a 



Fig. 65. Fig. 66. 



Fig. 64 shows left wing {a, l) of wasp in the act of twisting upon itself, the tip 

 of the wing describing a figiire-of-8 track (a, c, 5). From nature. — Or igino.l. 



Fios. 65 and 66 show right wing of blue-hottle fly rotating on its anterior 

 margin, and twisting to form double or figure-of-8 curves (a h, c d). From 

 nsiiwYQ.— Original. 



is most marked in the posterior or thin margin of the wing, the 

 anterior and thicker margin performing more the part of an axis. 

 As a result of this arrangement, the anterior or thick margin 

 cuts into the air quietly, and as it were by stealth, the posterior 

 one producing on all occasions a violent commotion, especially 

 perceptible if a flame be exposed behind the vibrating wing. 

 Indeed, it is a matter for surprise that the spiral conformation 

 of the pinion, and its spiral mode of action, should have 

 eluded observation so long ; and I shall be pardoned for 

 dilating upon the subject when I state my conviction that it 



