DR PETTIGKEW ON THE PHYSIOLOGY OF WINGS. 337 



of the planes of the wing occurred more especially at the end of the down and 

 up strokes. I inferred this from observing that the angle made by the wing 

 with the horizon is greater towards the termination than towards the middle of 

 the strokes. This could readily be ascertained by looking at the blur produced 

 by the oscillating wing edgewise, and this view revealed what is perhaps the 

 most important feature in wing movements, viz., that the tip of the wing during 

 its vibrations describes a scooped out (cde fig. 1) figure of 8 track as repre- 

 sented at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 of fig. 2. 



The Direction of the Stroke of the Wing in the Insect — ivhat Effective and what 

 Non-effective — the Kite-like Action of the Wing. — This view also showed that the 

 wing of the insect is made to vibrate in a more or less horizontal direction (figs. 

 3 and 5, page 338, Plate XI. fig. 4), in which respect it differs somewhat from 

 the wing of the bat and bird, these being worked more or less vertically (Plate 

 XI. figs. 5 and 6, and Plate XIV. figs. 18 and 19). The oblique action of the 

 pinion is necessary to avoid the resistance of the air during the up stroke, the 

 wing of the insect being in one piece, and having in many cases no adequate 

 apparatus for diminishing its area during its ascent. One great advantage 

 gained by the wing of the insect reversing its planes at the end of each stroke 

 consists in the great length of the effective stroke — the wing flying backwards 

 and forwards like a true kite, and tacking upon the air so suddenly as to 

 occupy very little either of time or space."" The period occupied by the wing 

 in reversing does not apparently amount to more than one-eighth of the time 

 taken up by one entire stroke, so that something like seven- eighths of the 

 area mapped out by the rapidly vibrating wing represents buoying area — the 

 remaining eighth slip. This, put in other words, simply means that in one 

 passage of the wing from behind forwards (down stroke) the pinion is effective 

 in seven- eighths of its course and non-effective in one-eighth, the same remark 

 being applicable to the passage of the wing from before backwards (up stroke). 



The Wing Attacks the Air at various Angles. — It is just possible that even less 

 than one-eighth is devoted to slip, from the fact that the wing when it is being- 

 reversed is slowed and applied to the air at an increased angle — a surface 

 which makes a large angle with the horizon, giving, when forced against the 

 air at a low speed, as much support as a similar surface whose inclination is 

 less, but whose speed is higher. As the wing attacks the air during the down 

 and up strokes at various angles, those angles being greatest when the wing 

 travels slowest, and least when the wing travels most rapidly, it follows that the 

 wing adapts itself to the resistance opposed to its passage by the air, and always 

 extracts the maximum of support from it. The wing, in this respect, differs 



* The movements of the wing somewhat resemble those of a sailing ship. The wing and ship 

 both tack upon the wind, and both change their tack or reverse abruptly. The changing of the tack 

 is moreover always accompanied by a slowing or diminution of the speed. 



VOL. XXVI. PART II. 4 S 



