PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
161 
deprived of this species of motion. We thus see that 
the wings of insects are in excess of their requirements, 
but in all cases the loss of -urface has to be made up 
by increase of speed. This is proved by the sound 
emitted by certain insects. Thus if the blue-bottle, which 
during ordinary flight makes a loud humming sound, has 
a portion of the posterior margins of its wings cut off, the 
sound it then emits is heightened in tone, showing that for 
the loss it has sustained it must vibrate its wings at a more 
rapid rate. Now, in all the experiments I have so far 
noticed it has been from the posterior margins of the wings 
we have detached portions, and it is only with this poste¬ 
rior portion we can take so much liberty, for if much of 
the ends of the wings be removed flight becomes impossible; 
and if the anterior margin is only notched across the strong 
nervures in the centre or near the base of the wing, the 
power of flight is at once destroyed, which circumstance 
shows very clearly that it is the anterior margin which is 
most essential to flight; in fact, if anything happens to 
the anterior margin flight is altogether impaired. We can 
also deprive insects of this power by covering the posterior 
margins with a coating which hardens as it dries, thus 
destroying the flexibility of the wing. 
The stroke of the wing in some insects is delivered 
in a vertical or down-and-up direction ; — in the ma¬ 
jority, however, it is in an oblique or nearly hori¬ 
zontal direction;—but all insects can and must often 
change the direction of the stroke. Those having the ver¬ 
tical stroke are all ample winged,—the wings of those 
with the oblique stroke being long and narrow. Now, it 
is very easy to understand that a downward impulse of the 
wings will give an upward impulse to the body, for we 
know that reaction is always equal and opposite to 
action (plate 1, fig. 3); but the more difficult ques¬ 
tion presents itself when we endeavour to explain 
the propelling power. Borelli (a celebrated physiolo¬ 
gist, who lived about 200 years ago, and was the first 
to carefully study the subject of flight), and all who 
follow his views, maintain that propulsion is effected 
by the wing striking the air, and causing it to rush with 
force in a backward direction; and in its passage under 
the wing, by bending the posterior margins upwards, it 
pushes the body forwards in an opposite direction. But 
experiment fails to prove this theory, for we cannot find 
that any current passes behind the wing in a backward 
direction, as we shall see when we come to notice the cur¬ 
rents. I am, therefore, inclined to believe on this, as 
on other points, with Professor Pettigrew, that the propel¬ 
ling power is the result of the construction of the wings, 
and their application upon the air. The fact of the 
wing being concave above and convex below, its screw¬ 
like configuration, and the way in which it strikes 
upon the air with its ever-varying angles, all render 
it especially adapted for giving horizontal motion ; so 
that when the wings strike vertically downwards and 
forwards, part of the force is expended on elevation, 
and part goes to the propulsion of the insect. This is 
readily perceived if you get a bird’s wing dried in an ex¬ 
panded position, or a large artificial wing constructed after 
the type of an insect’s wing. If you depress or elevate it vigor¬ 
ously, you will feel how it at once darts forwards in a curve. 
Most of the insects that are remarkable for the rapidity of 
their flight vibrate their wings in an oblique direction, 
this stroke giving less elevating and greater propelling 
power, for when the wings strike forwards and downwards 
the body is at once thrown forwards and upwards. (Plate 
I, fig. 6.) All who have spent any time watching a fly 
must often have been surprised by the apparently slight 
movement of the wings which is sufficient to launch it 
into space. 
I now turn to the currents. All wings give their 
effective stroke in a forward direction or towards the 
head of the insect. Even the vertical stroke is de¬ 
livered in a forward direction. When the wing is 
depressed (say vertically) in a forward direction, it 
uses the air under it as a fulcrum to obtain eleva¬ 
tion and propulsion. During its course, however, it 
drives the air before it, greatly increasing the pressure 
below and decreasing it above the wing. It, in fact, 
destroys the equilibrium of the air, which rushes in at once 
from all sides towards the path of the wing, creating 
currents of air, which flow with circling motions towards 
this place of least pressure. (Plate 1, fig. 6.) If you de¬ 
press the artificial wing to which I have referred before 
a gas jet, having the anterior margin next the flame, you 
will perceive that the flame is drawn towards the path of 
the wing; in other words, the wing induces a current at 
right angles to its path, which, as we shall hereafter see, 
it uses for its elevation. If you depress the wing with 
its posterior margin next the flame, the same thing occurs, 
and the flame is still drawn towards the path of the wing, 
which shows clearly I think that no current passes under 
the wing, or else the flame would be blown away from, 
instead of drawn towards, the wing. 
Let us now pass on to the consideration of the path 
of the wing, and in this we shall find one of the 
greatest difficulties connected with wing movements— 
viz., the elevation of the pinion. When a downward im¬ 
pulse is communicated to the wings, we know that a cor¬ 
responding upward impulse is communicated to the body 
