1872.] Natural and Artificial Flight. IgI 
sequently be removed without materially impairing either 
its form or its functions. This is proved by making sections 
in various directions, and by finding that in some instances 
as much as two-thirds of the wing may be lopped off with- 
out destroying the power of flight. Thus, in the summer 
of 1866, I removed the posterior two-thirds from either 
wing of a blow-fly, and still the insect flew, and flew well. 
The only difference I could perceive amounted to this, that 
the fly, while it could elevate itself perfectly, flew in circles, 
and had less of a forward motion than before the muti- 
lation. It had, in fact, lost propelling or driving power, 
the elevating or buoying power remaining the same. I took 
another blow- fly and removed the tip or outer third of 
either wing, and found that the driving power was the same 
as before the mutilation, while the elevating or buoying 
power was slightly diminished. ‘These experiments prove 
that the posterior or thin elastic margin of the wing is more 
especially engaged in propelling, the tip in elevating. The 
spiral nature of the pinion is most readily recognised when 
the wing is seen from behind and from beneath, and when 
it is foreshortened. It is also well marked in some of 
the long-winged oceanic birds when viewed from before, and 
cannot escape detection under any circumstances, if sought 
for,—the wing being essentially composed of a congeries of 
curves, remarkable alike for their apparent simplicity and 
the subtlety of their detail.” 
The Wing during its Action Reverses tts Planes, and describes 
a Figure-of-8 track in space-—The twisting or rotating 
of the wing on its long axis is particularly evident 
during extension and flexion in the bat and bird, and like- 
wise in the insect, especially the beetles, cockroaches, and 
others which fold their wings during repose. In these in 
extreme flexion the anterior or thick margin of the wing is 
directed downwards, and the posterior or thin one upwards. 
In the act of extension, however, the margins, in virtue of 
the wing rotating upon its long axis, reverse their positions, 
the anterior or thick margins describing a spiral course from 
below upwards, the posterior or thin margin describing a 
similar but opposite course from above downwards. ‘These 
conditions, I need scarcely observe, are reversed during 
flexion. The movements of the margins during flexion and 
extension may be represented 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 dire¢ted upwards and 
backwards during the return or up stroke. I infer this from 
