DP, PETTIGREW" ON THE PHYSIOLOGY OF WINGS. 327 



with a considerable degree of accuracy by a figure of 8 laid horizontally."" .... 

 It may likewise happen, though more rarely, that the anterior or thick margin 

 of the pinion may be directed upwards and backwards during the return or up 

 stroke. I infer this from having observed that the anterior margin of the wing 

 of the wasp (when the insect is fixed and the wings are being driven briskly) 

 is not unfrequently directed upwards and forwards at the beginning of the 

 down stroke, and upwards and backwards at the commencement of the up or 

 return stroke. A figure of 8, compressed laterally and placed obliquely with 

 its long axis running from left to right of the spectator, represents the move- 

 ment in question. The down and up strokes, as will be seen from this 

 account, cross each other, the wing smiting the air during its descent from 

 above, as in the bird and bat, and during its ascent from below, as in the flying 

 fish and boys' kite. The pinion thus acts as a helix or screw in a more or less 

 horizontal direction from behind forwards, and from before backwards ; but it 

 has a third function — it likewise acts as a screw in a nearly vertical direction 



from below upwards If the wing (of the larger domestic fly) be viewed 



during its vibrations from above, it will be found that the blur or impression 

 produced on the eye by its action is more or less concave. This is due to the 

 fact that the wing is spiral in its nature,t and because during its action it twists 

 upon itself in such a manner as to describe a double curve,! — the one curve 

 being directed upwards, the other downwards. The double curve referred to is 

 particularly evident in the flight of birds from the greater size of their wings. § 

 The wing, both when at rest and in motion, may not inaptly be compared to the 

 blade of an ordinary screw propellor as employed in navigation. || Thus the 

 general outline of the wing corresponds closely with the outline of the propellor, 

 and the track described by the wing in space is twisted upon itself propellor 

 fashion. The great velocity with which the wing is driven converts the 

 impression or blur IT into what is equivalent to a solid for the time being, in 

 the same way that the spokes of a wheel in violent motion, as is well under- 

 stood, completely occupy the space contained within the rim or circumference 



of the wheel From these remarks it will appear that not only the 



margins, but also the direction of the planes of the wing, are more or less 

 completely reversed at each complete flexion and extension ; and it is this 

 reversing, or screwing and unscrewing, which enables the wing to lay hold of 

 the air with such avidity during extension, and to disentangle itself with such 

 facility during flexion, — to present, in fact, a more or less concave, oblique, and 



* Op. cit., page 233, Diagram 5. Compare this diagram with. figs. 59 and 61 of Plate XV. 



t Op. cit., Plate XV. fig. 68. 



X Op. cit,. Plate XV. figs. 58 and 59 a a. Compare with a a of fig. 52. 



§ Op. cit, Plate XV. figs. 73 and 75 b a c. 



|| Op. cit, Plate XV. fig. 52 a a. 



f Op. cit, Plate XV. figs. 58 and 59. 



