162 
PROCEEDINGS OP THE PERTHSHIRE SOCIETY OP NATURAL SCIENCE. 
of the insect; but how is it that, during the upward move¬ 
ment of the wings, the converse does not take place, and 
the body become depressed to a like extent ? In birds we find 
that during the down-stroke the wing presents its greatest 
surface so as to obtain greatest leverage, while for the up¬ 
stroke it is by its construction able to flex or draw in, so 
that during the up-stroke a much smaller surface is pre¬ 
sented; and we further find that the feathers, by their 
construction and arrangement, form, during the down- 
stroke, an impervious barrier to the air, while during the 
up-stroke they separate, and the air passes through them 
as through a riddle. In the wing of the insect, however, 
no such arrangement is to be found, for the wings, 
being membraneous, are impervious to the air from 
either side. We shall, however, find, when we proceed 
further and trace the wing during the various phases of its 
evolution, that during the up as well as during the down- 
stroke elevating and propelling force is evolved. We must 
now endeavour to trace the wing throughout its course, 
and the more easily to obtain a thorough idea of this ques¬ 
tion, we will first of all notice what we may call—to use a 
Hibernian expression—stationary flight, or the motion of 
the wings of an insect which is held stationary, either by 
the hand or other means. The vibrations of the wings of 
an insect in this position form an elongated figure-of-8 
track in space. (Plate 1, fig. 4.) At the commence¬ 
ment of the down or forward-stroke the wing makes 
a forward angle of about 45 degrees with the horizon. 
It attains its highest speed about the centre of the 
stroke, or when it is at right angles to the body. The 
angle which the wing makes with the horizon at 
this point is less than it was at the commencement, 
owing to the resistance it experiences from the air. 
After passing the centre, the wing begins to decrease its 
speed, and owing to the anterior margin being stiffer it 
slows sooner than the posterior margin. This latter por¬ 
tion, at the end of the stroke, owing to the motion which 
has been communicated to it, twists round the anterior 
margin, and thus causes the wing to reverse its plane, and 
it then makes a backward angle of about 45 degrees, and 
is ready for the up-stroke. We have previously seen that 
the wing during its forward or down-stroke induces a cur¬ 
rent of air at right angles to its path : it now utilises this 
current, and rises upon it like a kite upon the wind. At 
the termination of the up-stroke the posterior-margin again 
twists round the anterior, again changing the plane of the 
wing, which is then ready to repeat the down-stroke. 
Thus there is a continuous movement, the down merging 
into the up-stroke, and vice versa. It is by a short 
movement of this description that many insects are 
able to poise themselves in the air, the forward and 
backward strokes combining to give elevation but no pro¬ 
pulsion. Many insects in this way rifle the corollas of 
flowers without alighting, and present the appearance, 
owing to the rapidity of their wing-movements, of being 
surrounded by a circle of motion. Those insects having 
ample wings and the vertical stroke cannot thus suspend 
themselves in space. 
Let us now pass on to the consideration of progres¬ 
sive flight. In the case we have just considered, the 
insect was held firmly. If, however, we had released 
our hold, it would instantly have darted off, and in 
that case, instead of making figure-of-8 curves, it would 
have made a waved track by the vibrations of its 
wings,—the difference being that in the former case the 
body of^the insect was stationary, while in the latter every 
vibration gives the body forward motion, and to carry the 
body k forwards is equal to carrying the wings backwards. 
The wings, therefore, in progressive flight, always make a 
forward anglejwith the horizon, and instead of going up- 
wards^and backwards go upwards and forwards. (Plate 
1, fig. 5.) All the movements, then, we have just 
noticed in regard to what we have termed station¬ 
ary flight are repeated in progressive flight, with the 
exception that the wing never reverses its plane, 
for when it makes about a right angle with the 
horizon Jt is seized by the current of air which it has by 
its down-stroke created, and, owing to the weight and 
forward travel of the body, the wing acts upon the air 
after the manner of a kite, and at once rises upwards and 
forwards upon this current, until it is again ready for a 
downward impulse. The momentum of the insect is the 
force by which the wing rises upon this current, just as 
the tension of the string is the force by which the kite 
mounts upon the wind. Thus, with the expenditure of a 
very trifling amount of force, the wing during its upward 
movement gives elevation as well as propulsion. Hitherto 
I have referred only to the power of horizontal flight, and 
all that need be said in regard to the endless gyrations 
performed by insects is that they are able to ascend, 
descend, or dart sideways as the case may be, either by 
the bending of the abdomen, thus displacing the centre of 
gravity by changing the plane of oscillation of the wings, 
or by the increase or diminution of the amplitude of the 
vibrations of the pinion. 
The more we consider the wing and its movements 
the greater must be our admiration of its powers. It 
is, in fact, a perfect mechanical instrument. As each 
of the various portions of the wing throughout its 
length oscillates at a different speed, so the angles and 
