Nov., 1889. 
THE FLIGHT OF BIUDS AND INSECTS. 
268 
force. This can be shown by a simple experiment. If we 
bend a piece of cane into a bow, cover it with paper so that it 
forms a crescent-shaped surface similar to that offered by the 
outstretched wings of the bird, we find that it will glide 
forwards on the air just as a bird does, and if it is furnished 
with screw propellers, like those of a steamship, driven by an 
elastic spring, it will continue this gliding motion till the 
spring is run down. 
Such a model as this makes very clear the two distinct 
functions of the wing : propulsion and support. The propul¬ 
sion must be the work of the long outer feathers, for the 
others, besides being unfitted for it by structure and arrange¬ 
ment, do not move up and down far or rapidly enough ; the 
support, which in this method of flight continues when the 
wings are merely extended, is especially the part of the 
continuous surface formed by the overlapping feathers nearer 
the body. It is, indeed, only for convenience that these move 
up and down at all; as our model shows, if the propelling- 
feathers could move up and down while the inner part of 
each wing was merely outstretched, the resulting flight would 
be the same. 
It will be noticed that in the smaller insects, which 
support themselves entirely by the motion of the wings, 
almost the whole of the wing-surface is at the end remote 
from the body, since only at the end of a long arm can it be 
moved through sufficient air in each stroke to make it effective. 
Where the support is afforded by the forward motion of the 
whole animal, as in the larger moths and all birds, every part 
of the wing serves equally for support, and the largest part of 
the surface is, therefore, near to the body, as being the position 
in which it can be supported with least material. 
Now, let me recapitulate. The typical insect flight, that 
is seen in perfection in small insects such as gnats, when 
hovering in still air, is effected by putting the wing-surfaces 
vertical, and moving them horizontally ; the support results 
from the motion of the wings, and ceases with it. The 
typical bird flight, on the other hand, is effected by placing 
the wing-surfaces nearly horizontal (but slightly higher in 
front), and moving them vertically, this propels, but does not 
support, and the support results from the forward motion, 
and lasts, even if the wings stop, till the forward motion 
ceases. But, on the one hand, every insect, when it moves 
forward, adopts to some degree the bird principle of flight ; 
and, on the other hand, most birds occasionally support them¬ 
selves, even in still air, without moving forwards, and, in this 
case, they place their bodies nearly vertically, and move their 
