Perhaps in the entire range of insect anatomy there is no point more
truly marvellous than the manner in which the respiratory system is
modified, in order to suit it to the peculiar requirements of its
owners.
In many ways the structure of the insects is wonderful enough. They are
gifted with muscles of extraordinary strength, and are yet destitute
of bones to which those muscles can be attached; they possess a
circulatory system, and are yet without a heart; they perform acts
involving the exercise of certain mental qualities, and are yet without
a brain. But, more remarkable still, they breathe atmospheric air
without the aid of lungs.
And this for a very good reason. It can be neither too often nor too
strongly insisted upon that, throughout animated nature, Structure
is in all cases subservient to Habit. If in any animal we find some
singular development in bodily form, we may be quite sure that there
is a peculiarity in the life-history which renders such development of
particular service, and so may often gain very complete information
with regard to the habits by a mere glance at external characteristics.
If, for example, the general shape is cylindrical, the toes webbed, and
the hair set closely against the body, we may safely conclude that the
animal is one intended for a life in the water. If the form is conical,
the limbs short, and the claws large and strong, that it is one which
burrows in the earth. If the jaws are large and massive, the teeth long
and sharply pointed, and the muscular power is concentrated principally
into the fore-parts of the body, that it is a beast of prey. And so on
with minor details.
And this rule holds equally good in the case of the insects, which are
devoid of lungs for the very sufficient reason that those organs are
necessarily weighty, and consequently unsuitable to the requirements
of beings which are in great measure creatures of air. In all animals
intended for a more or less aerial existence every particle of
superfluous weight must be dispensed with, in order that the strain
upon the muscles of flight may be reduced to the least possible
degree. Take the bats, and see how the skeleton has been attenuated
until it scarcely seems capable of affording the necessary rigidity
to the frame. Take the birds, and see how a large portion of the body
is occupied by supplementary air-cells, which permeate the very bones
themselves, and thus minimize the weight without detracting from the
strength. And so also with the insects, but in a different manner.
For in them the very lungs themselves are taken away, and replaced by a
respiratory system of great simplicity, and yet of wonderful intricacy,
which penetrates to every part of the structure, and simultaneously
aerates the whole of the blood contained in the body. In other words,
an insect is one large Lung.
* * * * *
If we take any moderately large insect, say a wasp or a hornet, we
can see, even with the naked eye, that a series of small spot-like
marks runs along either side of the body. These apparent spots, which
are generally eighteen or twenty in number, are in fact the apertures
through which air is admitted into the system, and are generally formed
in such a manner that no extraneous matter can by any possibility find
entrance. Sometimes they are furnished with a pair of horny lips, which
can be opened and closed at the will of the insect; in other cases
they are densely fringed with stiff, interlacing bristles, forming a
filter, which allows air, and air alone, to pass. But the apparatus, of
whatever character it may be, is always so wonderfully perfect in its
action that it has been found impossible to inject the body of a dead
insect with even so subtle a medium as spirits of wine, although the
subject was first immersed in the fluid, and then placed beneath the
receiver of an air-pump.
The apertures in question, which are technically known as “spiracles,”
communicate with two large breathing-tubes, or “tracheæ,” which extend
through the entire length of the body. From these main tubes are given
off innumerable branches, which run in all directions, and continually
divide and subdivide until a wonderfully intricate network is formed,
pervading every part of the structure, and penetrating even to the
antennæ and claws.
Physiologists tell us that if in the human frame the nerves, the
muscles, and the veins and arteries could be separated from one
another, while retaining their own relative positions, each would be
found to possess the perfect human form. In other words, there would be
the nerve-man, the muscle-man, and the blood-vessel-man, as well as the
bone-man which supplies the framework of the whole. In the same way we
may speak of the tracheal, or breathing-tube insect; for the two main
tubes and the endless ramifications of their branches, if they could
be detached from the surrounding tissues while themselves suffering no
displacement, would exhibit to us the form of the insect from which
they were taken, and that so exactly that in many cases we should
almost be able to recognize the species.
In the smaller branches of these air-vessels considerable variety
is to be found. Some retain their tubular character to their very
termination. Others assume a curious beaded form, dilating at short
intervals into small chambers; while yet others abruptly resolve
themselves into sac-like reservoirs, in which a comparatively large
quantity of air is stored up. From the larger vessels are thrown off
vast numbers of exceedingly delicate filaments, so small that a very
powerful microscope is necessary in order to detect them, which float
loosely in the blood, and furnish it with the constant supply of oxygen
necessary for its purification.
Now, we may well ask ourselves how it is that these tubes, which
are of almost inconceivable delicacy, should remain open during the
various movements of which the flexible body is capable. Why is it, for
instance, that the air-supply of the lower leg is not cut off when the
limb is bent at the knee-joint? or from the head, when that important
part of the frame is tucked away beneath the body? How does the
Earwig contrive to breathe while folding its wings by the aid of its
tail-forceps? or many of the Cocktail-beetles when curled up in their
peculiar attitude of repose?
The answer to these questions is simple enough, and may be discovered
by a glance at one of the most familiar of our own inventions—the
flexible gas-tube. This preserves its tubular form no matter to what
degree it may be bent or twisted, for coiled closely within it is a
spiral wire, which obliges the interior of the pipe to retain its
diameter almost unaltered alike when straight or curved. And as with
this, so with the tracheæ of the insect, whose walls are formed of a
double layer, the one lying inside the other, while between the two,
and surrounding the inner, is coiled a fine but very strong elastic
thread, whose convolutions allow the vessel to be bent in any required
direction without losing its cylindrical form. By the exercise of a
little care the anatomist can often unwind an inch or two of this
spiral thread from a single branch of the tracheæ of a tolerably large
insect, so closely is it coiled, and so elastic its character.
It will thus be seen that each expansion of the respiratory muscles
causes the air to rush to every part of the body, the entire bulk of
the blood being consequently aerated at each respiration. This fact is
a most important one, for, as it is not necessary that the blood should
be brought to a definite centre, as in the higher animals, before it
can be re-vivified, and then despatched through another series of
vessels upon its errand of invigorating the frame, the necessity for
a circulatory system is almost wholly at an end, and a large amount
of weight consequently dispensed with. Insects have neither veins nor
arteries, one principal vessel running along the back, and the blood
passing slowly through this, and flowing between the various organs of
the body until it again enters it at the opposite extremity to that
from which it emerged.
Nor is this all. With ourselves, as with the higher animals in general,
nearly one-half of the blood, the venous, is always effete and useless,
requiring to pass through the lungs before it can again be rendered
fit for service. When this is vivified and pumped back by the heart
into the system, that which was before arterial becomes venous in its
turn; and so on. But not in the case of the insects. The whole bulk of
their blood is arterial, if we may use the expression in speaking of
animals which do not possess a vascular system. In other words, being
incessantly vivified throughout the body, owing to the comprehensive
character of the respiratory apparatus, no portion of it becomes at
any time effete from the exhaustion of the contained oxygen. Blood
so thoroughly and continually aerated, therefore, can practically
perform double work, and need be far less in volume than in beings
whose circulation is conducted upon different principles. The tracheal
structure, consequently, while itself detracting from rather than
adding to the substance of the body, permits of the abolition, not
only of lungs, but also of veins and arteries and of a considerable
proportion of the blood, so that the weight of the insect is reduced to
the least possible degree.
There is yet another point to be considered, and that a very curious
and at present unexplained one. Upon careful investigation we find that
the tracheæ extend beyond the limits of the circulation, showing that
they must serve some secondary purpose in addition to that generally
attributed to them. For nature provides nothing in vain, and would not
without good and sufficient reason have carried the breathing-tubes
farther than necessary for their primary object of regenerating the
blood. As to what this purpose may be, however, we have no certain
knowledge, and can only conjecture that it is in some way connected
with the olfactory system. It is well known that the sense of scent
is in many insects very highly developed, enabling them to ascertain
the position of their food while yet at a considerable distance.
Burying-beetles and blowflies, for instance, will detect the faintest
odor of putrid carrion, and will wing their way without hesitation to
the spot whence it proceeds. Ivy-blossom, again, will attract almost
every butterfly and moth in the neighborhood, and this clearly by
reason of its peculiar fragrance.
It may be, therefore, that the perfection of the organs of scent in
insects is due to the fact that they are distributed throughout the
body, instead of being localized as is the case with animals higher in
the scale. That they must be connected with the respiratory apparatus
would seem, judging by analogy, to be indisputable, for, so far as we
know, an odor cannot be appreciated unless the air containing it be
allowed to pass more or less rapidly over the olfactory nerves. And in
no other part of an insect's structure could this requisite so well be
observed as in the tracheæ themselves, through which a stream of air is
continually passing, and which penetrate to the remotest parts of the
body.
With so wonderful a respiratory system, it naturally follows that an
insect must be particularly susceptible to the effects of any poisonous
vapor, which, being immediately carried to all parts of the body, must
speedily be attended by fatal results. And this is the case in a very
marked degree. A moth or beetle, which will live for hours, and even
days, after receiving an injury which would cause instant death to
a more highly organized being, will yet succumb in a few seconds to
the fumes of ether or chloroform, owing to the fact that the deadly
influence is simultaneously exerted upon all the nerve-centres of the
body, instead of being confined to one or two alone.
So much for the respiratory system of insects as a group. We have seen
how air is admitted into the body, how the entire bulk of the blood
is continuously aerated, and how every particle of needless weight
has carefully been dispensed with. There are many species, however,
whose mode of life renders necessary certain further developments, in
order that respiration may be carried on under circumstances which
would otherwise render it impossible. Such, for example, are the
various aquatic insects, which, while spending the greater part of
their existence beneath the surface of the water, must yet be enabled
to command a continual supply of atmospheric air. They are not, as a
rule, furnished with gills like the fish, for it is necessary that they
should be able to leave their ponds and streams at will, and become for
the time terrestrial or aerial beings, subject to the same conditions
as others of their class. But they are, nevertheless, provided with
certain modifications of structure, which enable them to breathe with
equal ease, whether submerged in the water, crawling upon the ground,
or flying through the air.
Even in these modifications there is considerable variety, dependent
in all cases upon the requirements of the individual species. The
Water-beetles, for instance, which must be able to lurk concealed among
the weeds, &c., until a victim comes within their reach, and then to
pursue and overtake it, carry down with them a supply of air in a
kind of reservoir, situated between the body and the wing-cases. The
former of these is concave and the latter convex, so that a chamber
of considerable size is formed, containing sufficient for their
requirements during a tolerably long period of time. And in these
insects the spiracles, instead of being situated along the sides of
the body, are placed upon the upper surface of the abdomen, so that
they open into the air-chamber itself, and allow the respiration to be
carried on without the slightest difficulty or inconvenience.
There is only one drawback to this arrangement, and that is, that the
increased buoyance prevents the insect from remaining beneath the water
excepting at the expense of active exertion, unless it can find some
submerged object to which to cling. Even this disadvantage, however,
is more apparent than real, for, while on the watch for prey, it is
necessary for the insect to remain as motionless as possible, and,
when engaged in swimming, the peculiar action of the oar-like limbs
neutralizes the tendency to rise towards the surface.
Upon an average, a water-beetle remains from fifteen to twenty
minutes without requiring to breathe; this period being capable of
considerable extension should occasion arise. I have forced one of
these insects, for instance, to stay beneath the surface for nearly
an hour and a half, by alarming it as often as it attempted to rise.
Generally speaking, however, before the first half hour is over, the
beetle allows itself to float to the surface, protrudes the tips of
the wing-cases, and expels the exhausted air from the cavity beneath
them; a fresh supply is then taken in, and the insect again dives, the
entire operation occupying barely a second of time.
The Water Scorpion affords us an instance of a perfectly different
structure.
Here we have a being, feeding upon living prey, which it must capture
for itself, and yet sluggish and slow of foot. By stratagem alone,
therefore, can it hope to succeed, and it accordingly lies hidden among
the dead leaves, sticks, &c., at the bottom of the water until some
luckless insect passes within reach of its jaw-like fore-limbs. But
this may not occur for hours, and it is imperatively necessary that no
alarm should be given by frequent journeys to the surface in search of
air. So, the extremity of the body is furnished with a curious organ
consisting of two long filaments, which are, in reality, tubular, and
which serve to convey air to the spiracles. The extreme tips of these
project slightly above the surface when the insect is at rest at the
bottom of the pond, so that respiration can be carried on without
difficulty, and without necessitating the slightest change of position.
A still more curious structure, although of very much the same
character, is afforded us by the grubs of the common Drone-fly. These
are inhabitants of the thickest and most fetid mud, dwelling entirely
beneath its surface, and consequently cut off from all personal
communication with the atmosphere. But from the end of the body
proceeds a long tube, which can be lengthened or shortened at will,
somewhat after the manner of a telescope, and which conveys air to the
spiracles just as do the tail filaments of the water scorpion. Unable
to change their position, these “rat-tailed maggots,” as they are
popularly called, are yet independent of any alteration in the depth of
the water above them, for the air-tube can be instantly regulated to
the required length, and so insure an uninterrupted supply of air.
Yet another system we find employed in the case of the grub of the
Dragon-fly, which stands almost alone among insects in its power of
extracting the necessary oxygen from the water itself. This is one of
the most rapacious of living beings, ever upon the watch for prey, and
securing its victims, not by stealth and fraud, but by open attack.
Its swimming powers, consequently, are of a very high order, and are
due to an organ which serves the double purpose of locomotion and
respiration, and which is one of the most wonderful pieces of structure
to be found in the whole of the insect world.
If a dragon-fly grub be even casually examined, a curious five-pointed
appendage will be noticed at the extremity of the body. If these
five points be carefully separated they will be seen to surround the
entrance to a tubular passage, of about the diameter of an ordinary
pin. This passage runs throughout almost the entire length of the body,
and, by the expansion and contraction of the abdominal muscles, can be
opened and closed at will.
When open, of course, it is instantly filled with water; when closed,
the contents are driven out with some little force. Consequently, the
action of the ejected fluid upon the surrounding water drives the
insect sharply forward, just as a sky-rocket rises into the air owing
to the action of the expelled gases upon the atmosphere. As soon as the
effect of the first stroke is at an end a second contraction of the
body takes place, and the operation is repeated as often as necessary.
The water, while in the swimming tube, however, is exhausted of its
oxygen, for the entrances to the respiratory system are inside instead
of outside the body, and act in much the same manner as do the gills
of a fish. The insect, therefore, is not obliged to visit the surface
of the water at all, and can continue to search for prey without
interruption.
Such are some of the many modifications brought about in insect
structure by the requirements of the respiratory organs alone. Each,
as will be noticed, is specially adapted to individual wants, and each
is absolutely perfect in its own way, insuring a continual supply of
oxygen for the purification of the blood, whatever the conditions under
which life may be carried on.—_Good Words._