RESPIRATION OF THE LARVA.
We well know, from the various melancholy accidents which have taken
place, that unless human beings have a constant supply of pure fresh
air they must perish. Many years ago, a number of unfortunate persons
were shut up in a narrow cell, called the Black Hole, at Calcutta,
where they could scarcely find room to stand, much less obtain air to
breathe. In one night the greater part of them died. More recently, as
some poor Irish were being conveyed in a steamer in stormy weather, the
captain, out of prudence and mistaken kindness, ordered the hatches
to be all battened down, so that fresh air was prevented from getting
in. A shocking spectacle was beheld soon afterwards:--in the close,
suffocating cabin lay a number of dead bodies, of men, women, and
children, all destroyed by the want of air. Now, precisely the same
fatal results take place if we treat larvæ in the same manner; from
which we learn that undoubtedly breathing is not only one of their
functions, but a most necessary and important one.
This may be easily proved. If the reader will procure about a dozen
caterpillars of any common kind, and put them into a dry phial, corking
them closely up, and if he leaves them in their prison of glass a
sufficient time, all will die, even though he may have supplied them,
on putting them in, with an abundance of food. Clearly, therefore,
they do not die of starvation, and we should be naturally disposed to
say they must have died of suffocation; that is, they died because
the function of breathing could not take place, as there was not the
necessary change of the air contained in the phial. But suppose that
other caterpillars were procured and put into another phial, with a
sufficiency of green food, the mouth being covered over with a piece
of muslin, or fine lace, what would then be the result? Simply that
they would live and thrive for as long a period as they received their
proper quantity and quality of food, and would pass through all their
stages of existence as comfortably as if in the open air. The reason
would be because the open fibres of the muslin, or lace, do not
interfere in the least with the requisite change of air for respiration.
Again, if after corking up the bottle for a short time, we were to
shake out all the caterpillars from it, and then put into it a little
water, in which quick-lime has been slaked--sold by chemists under
the name of lime-water,--we should find it become quite white and
milk-like. This would be, because, as the physiological chemist well
knows, the function of breathing causes carbonic acid gas to be poured
out of the body, no matter whether it is the body of an insect or an
elephant; and this gas has the property of turning lime-water of the
colour mentioned. Hence we have a second and convincing proof that
larvæ breathe--we need scarcely remind the reader that caterpillars
are larvæ--and more than this, that the function of breathing in them
resembles, so far as the discharge of this peculiar gas is concerned,
the same function, whether carried on in the human body or in that of
the most mighty and majestic of the beasts of the field.
The great chemist, Scheele, has experimented, somewhat in the manner
we have recommended, upon the breathing of larvæ. He found that
when caterpillars and maggots, to use the customary terms for these
varieties of larvæ, were confined in vessels containing only about
eleven cubic inches of atmospheric air, and so stopped as to preclude
the ingress of any fresh air whatever, although furnished with an
abundant supply of food, they soon died. When he put them into a still
smaller vessel, they died the sooner. In fact, precisely in proportion
to the purity, or to the amount of the air contained in the space in
which he confined them, were their lives prolonged or shortened. He
found on examining the air that it contained, as above stated, carbonic
acid gas.
[Illustration: _A Spiracle open._]
[Illustration: _The same shut._]
We may, therefore, consider the question as settled--that larvæ do
breathe. Strange it may appear to some, but neither in the larva nor in
any other form of its existence do insects breathe by the _mouth_. How,
then, it may be asked? We shall now describe their breathing apparatus
very briefly. Along each side of the body runs a delicate tube for the
conveyance of air, called a _trachea_, or when both are spoken of, the
_tracheæ_; these channels run underneath the muscles and skin, and
open to the air by little branch tubes, the mouths of which are called
_spiracles_. Besides these branch tubes which open to the air, the
large tubes send off a great number of smaller ones, which extend to
all parts of the body, penetrating even to the legs and wings. In some
insects, the spiracles, or breathing holes, are defended by a pair
of flaps, which swing to and fro, so as to let in the air, and close
upon it after each inspiration. By this simple means the breathing of
insects is carried on; the air enters the tubes, passes along them,
and is then conveyed to every part of the body; after which, having
fulfilled its duties, it escapes again. In man, and the higher animals,
there is a separate place for the air in the body, which we call the
lungs, and here the blood is conveyed to the air, in order to be
purified. But, as we have just seen, in insects it is very different,
for in them the air is conveyed by innumerable minute pipes to every
portion of the frame. Such is the breathing mechanism common in larvæ
which live in the air.
It will be readily imagined that those larvæ which dwell in the water
must be furnished somewhat differently, in order to enable _them_ to
breathe. And here, as we shall now find, there are some very curious
arrangements, in order to effect this object. They may be considered
under two divisions. First, contrivances for breathing air while the
larva is immersed in the water; and, second, apparatus for extracting
the dissolved oxygen gas necessary for breathing from the water.
[Illustration: _Breathing apparatus of an aquatic Larva._]
Let us select a few of the most singular instances under the first of
these divisions. "No better example," write the entertaining authors
of the Introduction to Entomology, "can be selected than the gnat. You
must have occasionally observed in tubs of rain-water, numerous little
wriggling worm-like animals, which frequently ascend to the surface,
there remain awhile, and then bending their head under the body,
rapidly sink again to the bottom. These are the larvæ of some species
of the genus just named; and if you take one out of the water and
examine it, you will perceive that it is furnished, near the end of the
body, with a singular organ, which varies in length according to the
species, and forms an angle with the last segment but one. The mouth
of this organ is funnel-shaped, and terminates in five points like a
star; and by this it is usually suspended at the surface of the water,
and preserves its communication with the atmosphere. In its interior
is a tube which is connected with the _tracheæ_, and terminates in
several openings, visible under a microscope at the mouth of the organ.
The points or rays of this mouth are used to close it when the animal
is disposed to sink in the water, and thus cut off its communication
with the atmosphere. When the animal is immersed, a globule of air
remains attached to the end of the tube, so that it is in fact of less
specific gravity than water, and it is not without some effort that it
descends to the bottom; but when it wishes to rise again, it has only
to unclose the tube, and it rises without an effort to the surface, and
remains suspended for any length of time. The extremity of the larva is
clothed with bunches of hairs, which are furnished with some repellent
material which prevents their becoming wet. It is this repellent
quality that probably causes a dimple or depression of the surface,
which, if you look narrowly, you will discover round the mouth of the
tube."
In summer time, the reader can scarcely fail of the gratification of
watching these most curious creatures, for they abound in ditches,
pools, and the stagnant waters of artificial fountains. They form a
very curious object under the microscope, and particularly in the
gas microscope, or solar microscope, where the image of the larvæ is
thrown, as in the case of the magic-lantern, upon a white sheet, or
wall. Their curious writhings to and fro, the energy they display in
lashing about their tails, so as to throw the few drops of water in
which they are imprisoned into mimic whirlpools, their occasional
dartings down to the bottom, and subsequent gentle rise up to the
surface, together with the terror into which countless minor occupants
of the same watery prison with themselves are thrown, when one of these
comparative giants plunges into the midst of them, form a spectacle as
unique as it is interesting and diverting. A sharp eye, and a phial and
cork, are the alone requisites for finding out these singular beings
on most fine days in summer. But a little while ago the writer was
amused to find, on a sunny day in June, the water of a small fountain
playing before his study window, thickly populated with these active
larvæ, and it became an amusement which seldom wearied, to watch their
fantastic evolutions, and to trace their changes from the larva through
the ulterior stage up to the perfect insect. To this, however, we shall
probably have occasion to revert before the conclusion of this work.
Another instance of a curious contrivance for assisting the breathing
of a larva, whose body is wholly immersed in water, may also be found
near home. Most probably the majority of our readers have seen, or
at any rate are well acquainted with the apparatus by which the
diver descends and works under water. Lest some should not, we may
mention that it consists of a water-tight dress, which covers the
diver from head to foot, while his head is enclosed in a helmet, or
hood, furnished with a pipe or hose, which is long enough to reach
to the surface of the water, and is there generally connected with
an air-pump, which forces down fresh air, and so enables the man to
breathe almost as comfortably as if he were walking on land, instead
of at the bottom of the sea. Let it be conceived now that there were
no air-pump at the top of the hose, but simply a buoy of cork to
keep it floating, and that the diver had the power of shortening or
lengthening the hose at his pleasure, we should then have a tolerably
fair representation of the contrivance with which Infinite Wisdom has
supplied the humble creatures of whom we are about to speak.
By carefully examining the water running from manure heaps, or water
which has become putrid, and is much loaded with mud and ooze, the
reader will pretty generally be successful in discovering a number of
the larvæ of a two-winged fly of the shape represented in the cut.
When discovered, the singularity of their structure will amply repay
him for the trifling unpleasantness attending the search after them.
After washing them in a little fair water, and putting them into a tall
ale-glass, or German beer-glass, which must be three parts filled with
water, they will be in the best condition for our examination. When
Réaumur first beheld them, he gave them the name of _Vers à queue de
rat_; that is, larvæ with a rat's tail; and when the reader looks at
them, as shown in the representation annexed, he will be inclined to
think with us, that a more appropriate title could not have been found.
[Illustration: _Rat-tailed Larvæ._]
On watching them they will be seen to wriggle about until they are
comfortably settled, darting from place to place, as if uneasy in their
new position and in the pure element. By-and-by, however, they become
reconciled to their lot, and lie perfectly quiet at the bottom of the
glass in the position represented. From the tail of each will now be
seen to rise a beautifully delicate and minute tube, which comes up
to the surface of the water, and here, whatever be the movements of
the larva below, it remains floating and free. Just as the diver may,
while his companions above are pouring down torrents of fresh air,
move about, and perform various duties under, water, so the larva
may move from one position to another, and assume almost any variety
of attitude; but its little air-pipe still retains its perpendicular
direction, and does not quit the surface of the fluid.
A singular experiment may now be tried with them. Let a little water be
poured into the glass by means of a funnel, the end of which dips under
the water so as not to give them unnecessary disturbance, and by this
means the level of the water in the glass becomes insensibly raised
half an inch or so. In a few seconds it will be found that these little
tails have all lengthened themselves, and are again at the surface
of the water, even at its higher level. More water may be poured in
until perhaps the level is raised three or four inches, and still the
same thing takes place, the larvæ still prolong their curious tails,
until it seems as if they would stretch to an indefinite length, for
they are already eight or ten times longer than the body! But, as we
might jestingly say--the longest _tail_ comes to an end at last; for,
on continuing to pour more water in, the larvæ stretch their tails to
the utmost, and then, finding the level of the water still above their
reach, they become uneasy, and all quit the bottom of the glass and
attach themselves to its sides.
[Illustration: _The Tail magnified._]
Let us now examine more minutely this very singular apparatus. By a
good lens we may readily do this, as the organ is very transparent,
permitting its internal structure to be beautifully seen. Placing the
creature in a convenient position for our examination, and in a good
light, it will be presently seen that the tail is composed in reality
of two tubes, of which one slides up and down within the other, just
like the tubes of a telescope (see cut). By this means the larva is
provided with an instrument capable of being made to reach to a great
length, and yet also possessing the capability of being packed up again
in a very small compass. The proper breathing tube is the small inner
one; at its extremity is a pair of spiracles, or breathing pores, and
surrounding these are placed five minute bristles, which are supposed
to act somewhat as a buoy, and to retain the tail in its position.
Where the tail joins on to the body of the larva, two tubes thrown into
folds are placed, which are connected with the breathing tubes of the
tail, at one end, and with the breathing pipes which run along the body
of the larva at the other. When the tail is pushed out to its farthest,
these zig-zag tubes are stretched quite straight; when it is drawn in
again, they are thrown into folds. Both these appearances are shown in
the plate. It has been supposed that the larva causes the tube to be
pushed out by forcing air into it, and thus causing the zig-zag tubes
to straighten themselves. Perhaps the tail is drawn in by the same
tubes contracting again, and thus folding together and dragging back
the tube to which they are connected.
[Illustration: _End of the Tail._]
We need scarcely stop to mention how useful is this admirable
contrivance to the insect. Having to seek for their food at the bottom
of ditches, drains, or puddles, they are necessarily exposed to the
risk of suffocation, unless fresh air can be conveyed to them; and
it is difficult for us to imagine how any apparatus could have been
contrived which would have adapted itself to all the varying depths
of water in which the insect must be constantly living, as it changes
from place to place. He must have been a clever engineer who could have
successfully met this difficulty. None of our present diving apparatus
does so. The organ we have been considering, the creation of an
Infinite Mind, small and despicable as it may seem in our eyes, fulfils
perfectly every function for which it was formed. It admits of free
movement from place to place, it admits, moreover, of free change from
one level to another in the fluid by which the larva is surrounded,
and it is at the entire disposal of the insect, which can, without the
smallest inconvenience, accommodate it to the various circumstances in
which it may be placed.
[Illustration: _Chameleon-fly Larva._]
Possibly the same phial in which was brought home the last-named
larvæ with the rat's tail, will furnish us with an equally elegant
instance of larva-respiration under water, in the case of the larva of
a fly, called the chameleon fly. This little creature, if it can be
found, and though not as common as the last, it may yet be detected,
in summer by the edge of ponds, &c., has truly elegant habits. The
great Swammerdam, who first described it, was lost in admiration at
it. The extremity of its body is furnished with a coronet of about
thirty elegantly feathered hairs. These are under the control of the
insect, and are capable of being folded up, so as to enclose a minute
bubble of air. When the larva is weary of remaining at the top of the
water, and wishes to dive to the still and cool depths below, it causes
these hairs so to fold up and close upon each other as to include the
air-bubble, so that it cannot escape. Thus furnished with a pearl in
its tail, it plunges downward, and thus breathes under water until the
purity of the captive air-bubble is impaired, when it returns to the
surface to repeat the same operations. The appearance of the little
bubble shining, with a brilliant silvery lustre under the water, is
very pleasing, and accounts for the enthusiastic expressions used
by Swammerdam, in speaking of the visible manifestation of the skill
of God in this insect. The fact of an insect coming to the surface
for air, and stealing away a portion, then diving down again into the
waters, is not uncommon in insect history, although few are possessed
of so elegant a contrivance for effecting it as the larva in question.
We must pass on to mention an instance or two of larvæ which, from
the peculiarity of their respiration, come under the second of our
divisions, that is, they live in the water, and do not breathe air, but
obtain the gas necessary for their respiration _from the water_, which
holds it in a state of solution. We shall first speak of the larva of
the common May-fly, the perfect insect of which is so well known to,
and highly prized by, anglers, under the various names of "duns" and
"drakes," than which in some streams no insect that flies is a better
bait to the unwary fish. Should the reader wish to examine them for
himself, they may be found in May or June, hiding themselves during
the day in the mud, or underneath stones, or in little holes in the
banks, or taking a sub-aqueous tour upon the bed of the brook, while
the clear and purling waters roll smoothly on above them. Their little
under-water cells are shown in the cuts, together with the larva.
[Illustration: _Cells of May-fly Larva._]
[Illustration: _Larva and section of its cell._]
Their organs for breathing the water, if the expression, which is in
some measure correct, may be admitted, are very like the gills of a
fish. But instead of forming a part of the structure of the mouth
and head, as those of the fish, in the larva they are placed along
its body, and are arranged in pairs. A delicate little air-tube
enters into each of these gills, and there branches out into a great
many directions; this tube is connected with the large trachea,
or breathing vessel of the insect, running along its body. The air
entering these gills, and thus being exposed to a large surface of
water containing dissolved gas, loses its impurities, and absorbs the
pure gas from the water, whence it is then conveyed to the various
parts of the body. In many species of these larvæ the whole series
of gills are moved to and fro incessantly, and so quickly as to be
scarcely perceptible to the eye. Some of them use them as oars,
moving with inconceivable rapidity, and being capable of darting both
backwards and forwards with great quickness. They have also the power
of leaping or springing in the water to a considerable distance; and
feed on minute aquatic larvæ, and on vegetable matters.
The only other instance of this kind that we shall quote will be that
of the terrible larva of the dragon-fly, whose deeds of insect murder
came before us on a former occasion. The apparatus provided for them
is very singular, and has been described with his usual accuracy by
the talented Réaumur. It exists in the tail of the insect, and may be
readily watched in action by adopting the following expedient:--Pour
into a small glass, in which the larva has been put, and which is
half full of water, a few drops of a solution of carmine, by means of
a little tube introduced into the water, and made to drop its contents
near the tail of the creature. Immediately it will be seen that the
coloured liquid is rapidly sucked in by the tail of the insect, and
then, after the expiration of a little time, is forced back again
in a tiny jet, with considerable violence, to a distance of several
inches.[H] Réaumur tells us, that if we hold the insect between the
finger and thumb gently, and let fall a drop of water on its tail,
we shall presently see it eagerly sucked in, the body of the insect
becoming sensibly larger; and by-and-by it is thrown out again.
Sometimes it raises its tail above the surface of the water, and,
whether for sport or not, it is hard to say, squirts the water in a
mimic jet to some distance.
The apparatus has been described as resembling the piston or plunger
of a force-pump, which moves to and fro within a cavity inside the
insect's body, and by that means draws in and forces out the water.
The tail is defended by a sort of coronet, composed of five hard
bristle-like pieces, which are under the control of the creature,
and may be opened and closed at pleasure. The larva appears capable
of inflicting a somewhat painful pinch by this contrivance, so that,
what with its wonderful mask, with its powerful squirt, and with its
tail thus armed, it would be difficult to select another insect better
prepared for battle with creatures of every aquatic tribe than it.
The action of the pumping apparatus sends it rapidly forward through
the water, somewhat like a paddle-wheel, or at least upon the same
principle as that contrivance, namely, the resistance offered by the
water to the stroke of this apparatus.