DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 325 



fish* as swimming in figure of 8, or looped curves. The wings of the insect, 

 bat, and bird, are also described and figured as executing figure of 8 movements 

 when the animals are hovering before an object, or when their bodies are 

 artificially fixed (page 336, figure 2) ;t the figure of 8, as I explained, being 

 opened out or unravelled when the animals are flying at a high horizontal 

 speed to form a looped and then a waved track (pages 341, 342, 344, and 345, 

 figures 10, 13, 14, and 15) .J 



The following brief passages from my memoir in the Transactions of the 

 Linnean Society^ will, I hope, serve to elucidate the peculiar figure of 8 move- 

 ments made by the wings in flight : — 



The Wing Twists and Untivists during its action. — " That the wing twists upon 

 itself structurally, not only in the insect, 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 repeated proofs.H 

 The twisting in question 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 

 insect. Indeed, it is 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 convic- 

 tion that it forms the fundamental and distinguishing feature in flight, and must 

 be taken into account by all those who seek to solve this most involved and 

 interesting problem by artificial means." The importance of the twisted confi- 

 guration or screw-like form of the wing cannot be over-estimated. That this 

 shape is intimately associated with flight is apparent from the fact that the 

 rowing feathers of the wing of the bird are every one of them distinctly spiral 

 in their nature ; in fact, one entire rowing feather is equivalent — morphologi- 

 cally and physiologically — to one entire insect wing. In the wing of the martin, 

 where the bones of the pinion are short and in some respects rudimentary, the 

 primary and secondary feathers are greatly developed, and banked up in such a 



a right-handed screw in the right knee-joint ; the anterior a right-handed, and the posterior a left-handed 

 screw in the left knee-joint. The movements which take place round these two combinations are 

 alternate, those round the anterior completing extension and commencing flexion, those round the 

 posterior completing flexion and commencing extension of the joint." 



* Op. tit., Diag. 2, page 204 ; Plate XV. fig. 76. 



t Op. tit, page 233, Diag. 5 ; Plate XV. fig. 61. 



X Op. tit, page 233, Diag. 6 j Plate XV. fig. 59. 



§ Op. tit, pages 231, 232, 233, and 234. 



|| Op. tit, Plate XV. figs. 68, 69, and 70. 



1 Op. tit., Plate XV. figs. 58, 61, 73, and 74. 



VOL. XXVI. PART. II. 4 P 



