Animals possess the power of feeling, and of effecting certain
movements, by the exercise of a muscular apparatus with which their
bodies are furnished. They are distinguished from the organisations of
the vegetable kingdom by the presence of these attributes. Every one
is aware, that when the child sees some strange and unknown object he
is observing start suddenly into motion, he will exclaim: 'It is
alive!' By this exclamation, he means to express his conviction that
the object is endowed with _animal_ life. Power of voluntary and
independent motion and animal organisation are associated together, as
inseparable and essentially connected ideas, by even the earliest
experience in the economy and ways of nature.
The animal faculty of voluntary motion, in almost every case, confers
upon the creature the ability to transfer its body from place to
place. In some animals, the weight of the body is sustained by
immersion in a fluid as dense as itself. It is then carried about with
very little expenditure of effort, either by the waving action of
vibratile cilia scattered over its external surface, or by the
oar-like movement of certain portions of its frame especially adapted
to the purpose. In other animals, the weight of the body rests
directly upon the ground, and has, therefore, to be lifted from place
to place by more powerful mechanical contrivances.
In the lowest forms of air-living animals, the body rests upon the
ground by numerous points of support; and when it moves, is wriggled
along piecemeal, one portion being pushed forward while the rest
remains stationary. The mode of progression which the little earthworm
adopts, is a familiar illustration of this style of proceeding. In the
higher forms of air-living animals, a freer and more commodious kind
of movement is provided for. The body itself is raised up from the
ground upon pointed columns, which are made to act as levers as well
as props. Observe, for instance, the tiger-beetle, as it runs swiftly
over the uneven surface of the path in search of its dinner, with its
eager antennæ thrust out in advance. Those six long and slender legs
that bear up the body of the insect, and still keep advancing in
regular alternate order, are steadied and worked by cords laid along
on the hollows and grooves of their own substance. While some of them
uphold the weight of the superincumbent body, the rest are thrown
forwards, as fresh and more advanced points of support on to which it
may be pulled. The running of the insect is a very ingenious and
beautiful adaptation of the principles of mechanism to the purposes of
life.
But in the insect organisation, a still more surprising display of
mechanical skill is made. A comparatively heavy body is not only
carried rapidly and conveniently along the surface of the ground, it
is also raised entirely up from it at pleasure, and transported
through lengthened distances, while resting upon nothing but the thin
transparent air. From the top of the central piece--technically termed
thoracic--of the insect's body, from which the legs descend, two or
more membraneous sails arise, which are able to beat the air by
repeated strokes, and to make it, consequently, uphold their own
weight, as well as that of the burden connected with them. These
lifting and sustaining sails are the insect's wings.
The wings of the insect are, however, of a nature altogether different
from the apparently analogous organs which the bird uses in flight.
The wings of the bird are merely altered fore-legs. Lift up the front
extremities of a quadruped, keep them asunder at their origins by bony
props, fit them with freer motions and stronger muscles, and cover
them with feathers, and they become wings in every essential
particular. In the insect, however, the case is altogether different.
The wings are not altered legs; they are superadded to the legs. The
insect has its fore-legs as well as its wings. The legs all descend
from the under surface of the thoracic piece, while the wings arise
from its upper surface. As the wings are flapping above during flight,
the unchanged legs are dangling below, in full complement. The wings
are, therefore, independent and additional organs. They have no
relation whatever to limbs, properly so called. But there are some
other portions of the animal economy with which they do connect
themselves, both by structure and function. The reader will hardly
guess what those wing-allied organs are.
There is a little fly, called the May-fly, which usually makes its
appearance in the month of August, and which visits the districts
watered by the Seine and the Marne in such abundance, that the
fishermen of these rivers believe it is showered down from heaven, and
accordingly call its living clouds, manna. Reaumur once saw the
May-flies descend in this region like thick snow-flakes, and so fast,
that the step on which he stood by the river's bank was covered by a
layer four inches thick in a few minutes. The insect itself is very
beautiful: it has four delicate, yellowish, lace-like wings, freckled
with brown spots, and three singular hair-like projections hanging out
beyond its tail. It never touches food during its mature life, but
leads a short and joyous existence. It dances over the surface of the
water for three or four hours, dropping its eggs as it flits, and then
disappears for ever. Myriads come forth about the hour of eight in the
evening; but by ten or eleven o'clock not a single straggler can be
found alive.
From the egg which the parent May-fly drops into the water, a
six-legged grub is very soon hatched. This grub proceeds forthwith to
excavate for himself a home in the soft bank of the river, below the
surface of the water, and there remains for two long years, feeding
upon the decaying matters of the mould. During this aquatic residence,
the little creature finds it necessary to breathe; and that he may do
so comfortably, notwithstanding his habits of seclusion, and his
constant immersion in fluid, he pushes out from his shoulders and back
a series of delicate little leaf-like plates. A branch of one of the
air-tubes of his body enters into each of these plates, and spreads
out into its substance. The plates are, in fact, gills--that is,
respiratory organs, fitted for breathing beneath the water. The
little fellow may be seen to wave them backwards and forwards with
incessant motion, as he churns up the fluid, to get out of it the
vital air which it contains.
When the grub of the May-fly has completed his two years of probation,
he comes out from his subterranean and subaqueous den, and rises to
the surface of the stream. By means of his flapping and then somewhat
enlarged gills, he half leaps and half flies to the nearest rush or
sedge he can perceive, and clings fast to it by means of his legs. He
then, by a clever twist of his little body, splits open his old fishy
skin, and slowly draws himself out, head, and body, and legs; and,
last of all, from some of those leafy gills he pulls a delicate
crumpled-up membrane, which soon dries and expands, and becomes
lace-netted and brown-fretted. The membrane which was shut up in the
gills of the aquatic creature, was really the rudiment of its now
perfected wings.
The wings of the insect are then a sort of external lungs, articulated
with the body by means of a movable joint, and made to subserve the
purposes of flight. Each wing is formed of a flattened bladder,
extended from the general skin of the body. The sides of this bladder
are pressed closely together, and would be in absolute contact but for
a series of branching rigid tubes that are spread out in the
intervening cavity. These tubes are air-vessels; their interiors are
lined with elastic, spirally-rolled threads, that serve to keep the
channels constantly open; and through these open channels the vital
atmosphere rushes with every movement of the membraneous organ. The
wing of the May-fly flapping in the air is a respiratory organ, of as
much importance to the wellbeing of the creature in its way, as the
gill-plate of its grub prototype is when vibrating under the water.
But the wing of the insect is not the only respiratory organ: its
entire body is one vast respiratory system, of which the wings are
offsets. The spirally-lined air-vessels run everywhere, and branch out
everywhere. The insect, in fact, circulates air instead of blood. As
the prick of the finest needle draws blood from the flesh of the
backboned creature, it draws air from the flesh of the insect. Who
will longer wonder, then, that the insect is so light? It is aerial in
its inner nature. Its arterial system is filled with the ethereal
atmosphere, as the more stolid creature's is with heavy blood.
If the reader has ever closely watched a large fly or bee, he will
have noticed that it has none of the respiratory movements that are so
familiar to him in the bodies of quadrupeds and birds. There is none
of that heaving of the chest, and out-and-in movement of the sides,
which constitute the visible phenomena of breathing. In the insect's
economy, no air enters by the usual inlet of the mouth. It all goes in
by means of small air-mouths placed along the sides of the body, and
exclusively appropriated to its reception. Squeezing the throat will
not choke an insect. In order to do this effectually, the sides of the
body, where the air-mouths are, must be smeared with oil.
In the vertebrated animals, the blood is driven through branching
tubes to receptacles of air placed within the chest; the air-channels
terminate in blood extremities, and the blood-vessels cover these as a
net-work. The mechanical act of respiration merely serves to change
the air contained within the air-receptacles. In the insects, this
entire process is reversed; the air is carried by branching tubes to
receptacles of blood scattered throughout the body; the blood-channels
terminate in blood-extremities, and a capillary net-work of
air-vessels is spread over these. Now, in the vertebrated creature,
the chest is merely the grand air-receptacle into which the blood is
sent to be aërated; while in the insect, the chest contains but its
own proportional share of the great air-system. In the latter case,
therefore, there is a great deal of available space, which would have
been, under other circumstances, filled with the respiratory
apparatus, but is now left free to be otherwise employed. The thoracic
cavity of the insect serves as a stowage for the bulky and powerful
muscles that are required to give energy to the legs and wings. The
portion of the body that is almost exclusively respiratory in other
animals, becomes almost as exclusively motor in insects. It holds in
its interior the chief portions of the cords by which the moving
levers and membranes are worked, and its outer surface is adorned by
those levers and membranes themselves. Both the legs and wings of the
insect are attached to the thoracic segment of its body.
The extraordinary powers of flight which insects possess are due to
the conjoined influences of the two conditions that have been
named--the lightness of their air-filled bodies, and the strength of
their chest-packed muscles. Where light air is circulated instead of
heavy blood, great vascularity serves only to make existence more
ethereal. Plethora probably takes the insect nearer to the skies,
instead of dragging it towards the dust. The hawk-moth, with its burly
body, may often be seen hovering gracefully, on quivering wings, over
some favourite flower, as if it were hung there on cords, while it
rifles it of its store of accumulated sweets by means of its long
unfolded tongue. The common house-fly makes 600 strokes every second
in its ordinary flight, and gets through five feet of space by means
of them; but when alarmed, it can increase the velocity of its
wing-strokes some five or six fold, and move through thirty-five feet
in the second. Kirby believed, that if the house-fly were made equal
to the horse in size, and had its muscular power increased in the same
proportion, it would be able to traverse the globe with the rapidity
of lightning. The dragon-fly often remains on the wing in pursuit of
its prey for hours at a stretch, and yet will sometimes baffle the
swallow by its speed, although that bird is calculated to be able to
move at the rate of a mile in a minute. But the dexterity of this
insect is even more surprising than its swiftness, for it is able to
do what no bird can: it is able to stop instantaneously in the midst
of its most rapid course, and change the direction of its flight,
going sideways or backwards, without altering the position of its
body.
As a general rule, insect wings that are intended for employment in
flight are transparent membranes, with the course of the air-tubes
marked out upon them as opaque nervures. These air-tubes, it will be
remembered, are lined by spires of dense cartilage; and hence it is
that they become nervures so well adapted to act like tent-lines in
keeping the expanded membranes stretched. In the dragon-flies, the
nervures are minutely netted for the sake of increased strength; in
the bees, the nervures are simply parallel. Most insects have two
pairs of these transparent membraneous wings; but in such as burrow,
one pair is converted into a dense leather-like case, under which the
other pair are folded away. In the flies, only one pair of wings can
be found at all, the other pair being changed into two little
club-shaped bodies, called balancers.
Butterflies and moths are the only insects that fly by means of opaque
wings; but in their case the opacity is apparent rather than real, for
it is caused by the presence of a very beautiful layer of coloured
scales spread evenly over the outer surface of the membranes. When
these scales are brushed off, membraneous wings of the ordinary
transparent character are disclosed. The scales are attached to the
membrane by little stems, like the quill-ends of feathers, and they
are arranged in overlapping rows. The variegated colours and patterns
of the insects are entirely due to them. If the wings of a butterfly
be pressed upon a surface of card-board covered with gum-water to the
extent of their own outlines, and be left there until the gum-water is
dry, the outer layer of scales may be rubbed off with a handkerchief,
and the double membranes and intervening nervures may be picked away
piecemeal with a needle's point, and there will remain upon the card a
most beautiful representation of the other surface of the wings, its
scales being all preserved by the gum in their natural positions. If
the outlines of the wings be carefully pencilled first, and the
gum-water be then delicately and evenly brushed on, just as far as the
outlines, a perfect and durable fac-simile, in all the original
variety of colour and marking, is procured, which needs only to have
the form of the body sketched in, to make it a very pretty and
accurate delineation of the insect.