PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
159 
vateand propel themselves in and through a medium of such 
tenuity. 
We cannot, however, further pursue this aspect of 
our subject, but must proceed to discuss the practical 
question we have before us, and after we see the way 
Nature has endowed some of her subjects with the power 
to travel through the air, we may, in conclusion, again 
return to this most interesting aspect of our theme. Now, 
in considering flight, the first question which meets us, and 
at which we must necessarily glance, is the consideration 
of those general laws,—outside the volant animal,—which 
affect flight. The first force, then, which presents itself 
for our consideration is the force of gravitation, which 
exercises so great an influence on so many of the problems 
of life—a force which attracts an unsupported body to the 
earth with a fall of about 16 feet per second. The flying 
animal has then, first of all, to raise itself into the air with 
this fores against it, and, when risen, it has to support 
itself against the downward tendency to which this force 
constantly subjects it. The second force which a bird or 
insect meets with and has to overcome is the resistance of 
the air, which, at a low velocity, is not very great, but 
which increases much more rapidly than the velocity in¬ 
creases, and which is very much augmented when a wind 
blows directly against the flying animal. These two forces 
then—viz., weight, and the resistance of the air—which at 
first sight seem so contrary to are nevertheless necessary 
to flight. Many seem to suppose the great essential to 
flight is lightness. This, however, is a common error. 
The balloon rises because it is lighter than the air, and 
thus escapes the grasp of gravitation, but the difference 
between a balloon and a flying animal is that the former 
is driven about by every wind, while the latter has a power 
residing in itself on account of its weight by which it is 
able to steer and regulate its course. All volant animals 
then possess weight, and are as heavy, bulk for bulk, as 
any other animals. Some may point to the air-cells and 
hollow bones in many birds, but it is now I think very 
generally believed that these add nothing to the capabilities 
of flight of these birds. The hollow bones are considered 
as being formed for strength, and the air-cells are such 
that the advantages they are supposed to confer are 
certainly not very great. iSome believe that the air-sacs 
are adjuncts of the lungs, and assist in aerating the blood. 
That hollow bones are not necessary to flight I think is 
clearly proved by such splendid flying birds as the swifts, 
martins, and snipes having bones filled with marrow; and 
we find that large air-cells are possessed by other animals, 
such as the orang outang, whose powers of flight are cer¬ 
tainly not very striking. A certain amount of weight, 
then, is necessary to flight;—it not only gives balance, but, 
as we shall presently see, it gives sustaining and elevating 
power, for when the flying animal has attained momentum 
its weight relieves the exertion falling upon the wings in 
attaining motion, and thus accounts for the prolonged 
flight of many insects and birds. 
We now come to consider the wings of insects, 
and must first notice their construction. The wings 
or organs of flight of insects vary veiy much in 
form, strength, and opacity. Some insects possess 
two pairs and some only one pair; some are delicate 
transparent membranes, such as those of the common flies, 
dragon flies, bees, &c.; others are covered with minute 
hairs, such as in the caddis flies (Pliryganidce), &c.; others, 
again, are opaque, and often of the most beautiful patterns 
and colours, as in the butterflies and moths. In some, 
such as the beetles, the front pair of wings are modified, 
being of a hard and horny nature, and serve as a sheath or 
cover for the hind pair of wings, but they still play an im¬ 
portant part in flight. When we take up a wing, and, 
holding it between ourselves and the gas, look through it, 
whether it be the opaque wing of a butterfly or the trans¬ 
parent wing of a common fly, we notice that it is crossed 
by bars known to entomologists as uervures; and in some 
parts it is recrossed, or these bars are connected together 
by other bars which are termed nervules. These bars, 
when seen under the microscope, are usually found to be 
double tubes. Some have thought that one contained air, 
so as to give lightness, and the other contained a fluid, to 
give pliability to the wing;—others, again, have thought 
these tubes were connected with the respiratory system. 
Whether they have a twofold purpose or not we cannot 
tell, but that the main object is to give rigidity com¬ 
bined with elasticity to the wing is certain. If we 
examine a variety of wings, large and small, we shall find 
they are all built up after one general plan. You will 
notice that they are all strongest at the base or thoracic 
end of the wing, and along the anterior margin of the wing, 
getting finer and finer towards the posterior and outside 
margins, (Plate I., fig. 1.) The nervures are strongest in 
the beetles, where the body is heavy and the wings small, 
and decrease in strength and number as these conditions 
are receded from, until in some of the Chalcidce no nervures 
are present. Another very important point is the confor¬ 
mation of the wing. If you pull one of the wings off a fly 
or bee, and hold it with the anterior margin on a level 
with the eye, you will notice that the wing possesses a 
twisted or screw-like surface; in fact, if you take one of 
the primary wing-feathers of a bird, you have in conforma¬ 
tion an exact representation of an insect’s wing. 
