192 Natural and Artificial Flight. [April, 
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 for- 
wards 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 movement 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 asa 
helix or screw in a more or less horizontal dire¢tion from 
behind forwards, and from before backwards; but it has 
a third fun¢tion—it likewise a¢ts as a screw in a nearly ver- 
tical 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 (Fig. 5, aa’). 
Fic. 5. 
Blur or impression produced on the eye by the rapidly oscillating 
wing of the Blow-Fly when the inse¢t is progressing at a high 
speed. Seen from above and from the side a a’ represents the 
waved track made by the wing in progressive flight. 
This is due to the fact that the wing is spiral in its nature, 
and because during its action it twists upon itself in sucha 
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 (Fig. 4, ba,ac). The wing, 
both when at rest and in motion, may not inaptly be com- 
pared to the blade of an ordinary screw propeller as 
employed in navigation. Thus the general outline of the 
wing corresponds closely with the outline of the propeller, 
and the track described by the wing in space is twisted upon 
itself propeller fashion. The great velocity with which the 
wing is driven converts the impression or blur 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 
