DR, PETTIGREW ON THE PHYSIOLOGY OF WINGS. 335 



When the wing acts in this manner, it is the under or ventral surface which is 

 effective both during the forward and backward strokes. The wing, during 

 the back stroke occasions very little friction, from its being placed in a more 

 or less horizontal position — this position being favourable to its affording a 

 maximum of support. The upper and under surfaces of the wing are applied 

 to the air alternately, more particularly when the insect is fixed, or when it is 

 hovering in one spot. When it is flying at a high horizontal speed, and when 

 the wing is made to oscillate in a slightly vertical direction, as in the butterfly 

 (figures 29, 30, 31, 32, 33, and 34, page 360) and dragon-fly (figures 35, 36, 37, 

 and 38, page 361), it is the under or concave surface of the pinion which does 

 the principal part of the work, this attacking the air both during the down or 

 forward stroke and the up or backward stroke, like a boy's kite, as explained 

 at pages 349 and 350, figures 16 and 17. The direction of the stroke varies 

 slightly according to circumstances, but it will be quite proper to assume that 

 the wing of the insect is made to vibrate in a more or less horizontal direction, 

 and that of the bird and bat in a more or less vertical direction. By a slight 

 alteration in the position of the body, or by a rotation of the wing in the 

 direction of its length, the vertical direction of the stroke is converted into 

 a horizontal direction, and vice versa. The facility with which the direction of 

 the stroke is changed is greatest in insects ; it is not uncommon to see them 

 elevate themselves by a figure of 8 horizontal screwing motion, and then, sud- 

 denly changing the horizontal screwing into a more vertical one, to dart rapidly 

 forward in a curved line. The horizontal screwing movement is represented at 

 figures 2, 3, 4, 5, 6, 7, and 10, pages 336, 338, 340, and 341 ; and the vertical 

 screwing at figures 12 and 13, page 342. The horizontal action of the insect's 

 wing is described at pages from 336 to 341 inclusive, and the vertical action at 

 pages from 347 to 355 inclusive. The vertical action of the bat and bird's wing 

 is described at page 342, and at pages from 366 to 397 inclusive. Whether the 

 wing is made to vibrate vertically or horizontally, it, practically speaking, in 

 progressive flight, strikes doivnwards and forwards during the down stroke, and 

 upwards and forwards during the up stroke, as fully explained at pages 344 

 and 345. 



Compound Rotation of the Wing. — The wing during its vibration rotates 

 upon two separate centres, the tip rotating around the root of the wing as an 

 axis (short axis of wing), the posterior margin rotating around the anterior margin 

 (long axis of wing). This compound rotation goes on throughout the entire 

 down and up strokes, and is intimately associated with the power which the 

 wing enjoys of alternately seizing and evading the air. 



The Wing inclined Fomvards at the End of the Doivn Stroke and Backwards 

 at the End of the Up Stroke. — I had my attention first strongly directed to the 

 screwing figure of 8 action of the wing by closely observing the twisting figure 



