Oct., 1889. 
THE FLIGHT OF BIRDS AND INSECTS 
223 
revolving vanes of our model. These backward and forward 
motions are not the best, but tliev are for the animal the best 
possible. 
In the actual insect, then, the wings must move backwards 
and forwards, not round and round, but the motion is still 
horizontal, not vertical like that of a bird’s wings ; the white 
blur seen when a fly balances itself in the air clearly shows 
this fact. As the wing surfaces must always be so inclined 
that the upper edge is in front, this inclination must change 
with the direction of the motion. In the insect this is 
managed automatically by the simple device of placing the 
surfaces vertical, and making the upper edge stiff, the lower 
thin and flexible ; the resistance of the air to their motion 
then sets them in the required position, since it forces the 
flexible lower edge further back than the stiff upper one. 
If we make a pair of wings by stretching thin paper over 
two thin strips of cane tapered to a point, and bent nearly at 
right angles near one end, these wings will resemble those of 
the insect in being stiff all along one edge and flexible along 
the other; and if we attach these to a frame, so that the 
cane edges are uppermost and horizontal, while the planes of 
the paper are vertical, they will be in a position like the 
insect’s when it flies. If, by means of twisted elastic cord 
driving a crank and connecting-rod, we cause the strips of 
cane to move horizontally backwards and forwards like the 
oars of a boat, it will be seen that at each stroke the surface 
of the wing changes from a vertical to an inclined position. 
If this model is placed on one pan of a balance, and partly 
balanced by weight in the other pan, the pan containing it 
will rise as soon as the wings begin to move, showing, as 
might be expected, that these inclined surfaces tend to rise. 
It will be seen that the principle of this model, and of the 
insect which it imitates, is really the same as that of the 
models with rotating wings, and that the adoption in the 
insect of an oscillatory instead of a rotary motion is a 
necessity not for flight but for growth; but it must not be 
supposed that it is as good an arrangement considered 
mechanically. All oscillatory motions are wasteful, and this 
waste becomes an important quantity whenever, from the 
nature or lightness of the structure, there is a considerable 
want of rigidity. In the insect the oscillatory movement is 
the cause of great waste of power. At the beginning and at 
the end of each stroke the wing is moving too slowly to bend 
the surface into the slanting position required ; these parts of 
the stroke, therefore, exert no supporting power at all, and 
there are further losses from the want of sufficient rigidity in 
the nervures, which shortens the stroke without any corre¬ 
sponding diminution of work. So that while a set of sloping 
