sides of the abdomen of the larvæ and pupæ of the _Ephemeræ_, whose
history you found so interesting[241]. In them these organs wear
much the appearance of _gills_. In the different species they vary
both in their number and structure. With regard to their number,
some have only _six_ pair of them, while others have _seven_. In
their _structure_ the variations are more numerous, and sometimes
present to the admiring physiologist very beautiful forms[242].
They usually consist of two branches, but occasionally are single,
with one part folding over the other, as in one figured by Reaumur,
which precisely resembles the leaf of some plant, the air-vessels or
_bronchiæ_ in connexion with the _tracheæ_ branching and traversing
it in all directions, like the veins of leaves[243]. The double ones
differ in form. In the larva and pupa of _Ephemera vulgata_ there
are _six_ of these double false gills on each side of the abdomen,
the three last segments being without them; each branch consists
of a long fusiform piece, rather tumid and terminating in a point,
which is fringed on each side with a number of flattish filaments,
blunt at the end. An air-vessel from the _trachea_ enters the gill at
its base; is first divided into two larger branches, each of which
enters a branch of the false gill. These branches send forth on each
side numerous lesser ramifications, one of which enters each of the
filaments[244]. In another species (_E. vespertina_) each false gill
presents the appearance of a pair of ovate leaves with a long acumen,
and the air-vessels represent the midrib of the leaf, with veins
branching from it on each side[245]; and, to name no more, in _E.
fusco-grisea_, one branch represents the leaf of a _Begonia_, the
sides not being symmetrical, with its veins, while the other consists
only of numerous branching filaments[246]. In other aquatic larvæ, as
in that of the common May-fly (_Sialis lutaria_), these appendages
consist of several joints[247].
By the above apparatus these aquatic animals are enabled to separate
the air from the water, as the fish by their gills; but how this
separation is made has not been precisely explained. The false gills
in many species are kept in continual and intense agitation. When
they move briskly to one side, Reaumur conjectures they may receive
the air, and when they return back they may emit it[248]. This brisk
motion probably disengages it from the water. In many species, when
in repose, they are laid upon the back of the animal[249], but in
others they are not[250].
The larvæ of the _Agrionidæ_ appear to respire like those of the
_Ephemeræ_, &c. by means of long foliaceous laminæ or false gills
filled with air-vessels; but instead of being _ventral_, they
proceed from the _anus_. They are three in number, one dorsal and
two lateral, perpendicular to the horizon, of a lanceolate shape,
beautifully veined, with a longitudinal middle nervure, from which
others diverge towards the margin, which are probably _bronchiæ_.
They are used by the animal, which swims like a fish, as fins, but it
does not appear to imbibe the water like the other _Libellulinæ_, nor
to propel itself by ejecting it,--a circumstance which furnishes an
additional argument for the more received opinion, that this action
in them is for the purpose of respiration as much as for motion[251].
The larvæ and pupæ of the _Libellulinæ_, receive the water and
air that they respire by a large anal aperture, which is closed
at the will of the animal by five hard, moveable, triangular,
concavo-convex pieces, all very acute and fringed with hairs. These
pieces are placed so that there is one above, which is the largest
of all; one on each side, which are the smallest, and two below;
when these are closed they form together a conical point[252].
Sometimes only three of these pieces are conspicuous[253]: three
other cartilaginous pieces, resembling the valves of a bivalve shell,
close the passage within the pointed pieces[254]. At this orifice the
water is received; and when, by an internal process to be described
afterwards, it has parted with its oxygen, is again expelled.
Under this head I shall mention a fact which may be connected with
respiration of the insects concerned. In dissecting a moth related
to _Catocala Pronuba_, but I do not recollect the particular
species,--at the base of the abdomen of the male I discovered two
bunches of long fawn-coloured parallel hairs, planted each in an oval
plate, plane above, but below convex and fleshy; while the plates
remained attached to the insect, they appeared to have a distinct
pulsation. The hairs, which are about half an inch long, diverge a
little, and form a tuft not very unlike a shaving-brush[255]. I have
not since met with this species, but I have preserved the brush and
scale. Somewhere in Bonnet's works, but I do not recollect where, I
have since found mention of a similar fact in another moth.
II. Having considered the _external_ respiratory organs of insects,
by which the air is _received_, we are next to consider the
_internal_ ones, by which it is _distributed_. These are _gills_;
_tracheæ_ and _bronchiæ_; and _sacs_ or _pouches_[256].
i. Gills (_Branchiæ_[257]). Having lately described what may be
denominated _false_ gills, or branchiform appendages, I shall now
call your attention to what may be denominated _true_ ones, which are
peculiar to the _Arachnida_ Class: but what is remarkable, the animals
that breathe by them are very rarely inhabitants of the water, so that
their functions cannot be perfectly analogous to those of fishes.
In the _Scorpion_, on each side of the four first ventral segments a
spiracle may be discovered, which has no _lip_ as in other insects,
but is merely a circular _orifice_. These orifices do not lead to
_tracheæ_ or _vesicles_, but to _true gills_, which are situated
below a muscular web which clothes the internal surface of the
crust. Each gill consists of many semicircular very thin plates, of
a dead milky white, which are connected together at the dorsal end
like the leaves of a book. There appear to be more than _twenty_ of
these leaves, which when strongly magnified look transparent and
destitute of any vessels. Each gill is fastened at the back to the
spiracle[258]. In the _spiders_ also, gills are discoverable, but
differently circumstanced. On the under side of the abdomen, near
the base, is a transverse depression, on each side of which is a
longitudinal opening leading to a cavity, which is covered from above
by a cartilaginous plate. In this cavity is situated a true gill,
which is white, triangular, and covered with a fine skin; the leaves
of this gill are far more numerous and much finer and softer than
those of the gills of the scorpion. On account of their softness
they have often the appearance of a slimy skin; but their laminated
structure shows itself very clearly in old specimens, and in such as
have been immersed in boiling water[259].
ii. _Tracheæ_ and _Bronchiæ_[260]. Parallel with each side of the body
of most _insects_ and extending its whole length, run _two_ cylindrical
tubes[261], which communicate with the spiracles[262], and from which
issue, at points opposite to those organs, other tubes which ramify
_ad infinitum_, and are distributed to every part of the body[263].
The first of these tubes are called the _tracheæ_ and the latter the
_bronchiæ_. This structure appears, however, not to be universal: it
is to be found in caterpillars and many _Dipterous_ larvæ; but in that
of the rhinoceros-beetle and other Lamellicorns, the _bronchiæ_ branch
_directly_ from the spiracle, the bottom or interior mouth of which is
lined by a membrane from which they proceed[263]: something similar
has been observed to take place in many insects in other states, as
the common cockchafer[264]; in the pupa of _Smerinthus Populi_[265];
in the _Cicadæ_[266]; in the Locust tribe[267]; and many others. In
the _Cossus_, or larva of the great goat-moth, the _trachea_ commences
with the first spiracle, and finishes a little beyond the last, after
which it diminishes considerably in diameter, and terminates in several
branches or _bronchiæ_, which proceed to the anal extremity of the
body[268]. The _bronchiæ_ which originate from the _tracheæ_ in the
vicinity of each spiracle, may be considered as consisting in general
of _three_ packets;--_dorsal_ ones, which are distributed to the back
and sides of the animal; _visceral_ ones, which enter the cavity of
the body, and are lost amongst the viscera and the caul; and _ventral_
ones, which dipping from the _tracheæ_ overrun the lower part of the
sides and belly[269].
The _tracheæ_ and _bronchiæ_ consist of _three_ tunics[270]: the
_first_ or external one is a thickish membrane, strengthened by
a vast number of fibres or vessels, which form round it a number
of irregular circles; the _second_ is a membrane more thin and
transparent, without a vascular covering[271]; the _third_ is formed
of a cartilaginous thread running in a spiral direction, which may
be easily unwound[272]. This structure gives a great elasticity to
these organs, so that they are capable of considerable tension,
after which they return to their usual length[273]. The _Bronchiæ_
are cylindrical or slightly conical, insensibly diminishing in size
as they leave the trunk, in which they originate. In larvæ, after
losing their spiral fibre, they appear to terminate in membrane,
but in perfect insects they pass into vesicles[274]. In the
_Cossus_ the _trachea_ is flattened, and in every segment, except
the first and two last, is bound by a fleshy cord four or five
times as thick as its threads. Where this occurs, there is a slight
constriction,--probably here is a sphincter, by the contraction of
which Lyonet supposes the _trachea_ may be shut when it is necessary
to stop the passage of the air, and direct it to any particular
point[275]. The structure here described is admirably adapted for the
purpose it is intended to serve; for had these vessels been composed
of _membrane_, they could not possibly have been prevented from
collapsing; but by the intervention of a spiral cartilaginous thread
this accident is effectually guarded against, and the necessary
tension of the tubes provided for. However violent the contortions
of the insect, however small the diameter of these vessels, they are
sure to remain constantly open, and pervious to the air. And by this
circumstance they may be always distinguished from the other organs
of the animal, and likewise by their pearly or silvery hue, for from
being constantly filled with air, these tubes, when viewed under a
powerful microscope in a recently dissected insect, present a most
beautiful and brilliant appearance, resembling a branching tree of
highly polished silver or pearl:--though sometimes they are blue, or
of a lead colour, and sometimes assume a tint of gold. In the dead
insect the larger tubes soon turn brown, but the finer ones preserve
their lustre several weeks[276]. The ramifications of the tracheal
tree may be seen without dissection through the transparent skin of
the common louse[277] and most of the thin skinned larvæ.
You will not expect to view in this way the minuter ramifications of
the _bronchiæ_, when I have mentioned their number and incredible
smallness. Nothing but the scalpel of a Lyonet and the most powerful
lenses are adequate to trace the extremities of these vessels; and
even with every help, they at last become so inconceivably slender as
to elude the most piercing sight. That illustrious anatomist found
that the two _tracheæ_ of the larva of the _Cossus_ gave birth to
236 bronchial tubes, and that these ramify into no less than 1336
smaller tubes, to which, if 232, the number of the detached bronchiæ,
be added, the whole will amount to 1804 branches[278]. Surprising
as this number may appear, it is not greater than we may readily
conceive to be necessary for communicating with so many different
parts. For, like the arterial and venous trees, which convey and
return the blood to and from every part of the body in vertebrate
animals, the _bronchiæ_ are not only carried along the intestines and
spinal marrow, each ganglion of which they penetrate and fill, but
they are distributed also to the skin and every organ of the body,
entering and traversing the legs and wings, the eyes, antennæ, and
palpi, and accompanying the most minute nerves through their whole
course[279]. How essential to the existence of the animal must the
element be that is thus anxiously conveyed by a thousand channels,
so exquisitely formed, to every minute part and portion of it! Upon
considering this wonderful apparatus we may well exclaim, _This hath
GOD wrought, and this is the work of his hands_.
Though in general there is only a _pair_ of _tracheæ_, yet in
some larvæ a larger number have been discovered. In those of the
_Libellulinæ_ there are _six_. According to M. Cuvier, Reaumur,
who mentions only _four_, overlooked the two lateral ones that are
connected with the spiracles[280]. The reason of this and other parts
of their internal structure I shall explain under the next head.
In the grub of the gad-flies of the horse (_Gasterophili_,) Mr. B.
Clark discovered _eight_ longitudinal _tracheæ_,--_six_ arranged in a
circle and _two_ minute ones, which appeared to him to terminate in a
pair of external nipples (spiracles) in the neck of the animal[281].
This is a singular anomaly, as the other _Œstridæ_ have only a _pair_
of _tracheæ_[282].
iii. _Respiratory Sacs or Pouches._ Besides their _tracheæ_ and
_bronchiæ_, many insects are furnished with reservoirs for the air,
under the form of sacs, pouches, or vesicles. These are commonly formed
by the bronchial tubes being dilated at intervals, especially in the
abdomen, into oblong inflated vesicles; from which other bronchial
tubes diverge, and again at intervals expand into smaller vesicles, so
as to exhibit no unapt resemblance--as Swammerdam has observed with
respect to those of the rhinoceros-beetle--to a specimen of _Fucus
vesiculosus_. Cuvier compares them in the Lamellicorn beetles in
general to a tree very thickly laden with leaves[283]; and Chabrier
observes that they particularly occur in the intestinal canal[284].
This structure of the pulmonary organs may be seen also in the common
hive-bee, and other _Hymenoptera_; but the vesicles are less numerous,
and those at the base of the abdomen much larger than the rest[285].
These vesicles, by a very rough dissection, may be distinctly seen in
the abdomen of the cockchafer, which appears to be almost filled with
them. Not being composed of cartilaginous rings like the air-tubes,
but of mere membrane, if a pin pierces one, the air that inflates it
escapes, and it collapses. In the larva of a little gnat (_Corethra
culiciformis_) the _tracheæ_ appear to proceed from a pair of oblong
vesicles of considerable size[286] in the trunk, and towards the anus
they form two other smaller ones[287],--upon piercing the former, De
Geer observed a considerable quantity of air to make its escape[288].
Another species, probably of the same genus, described by Reaumur,
exhibits something similar[289].
But one of the most remarkable structures, in this respect, is to
be seen in the larva and pupa of the dragon-flies (_Libellulina_).
I have before noticed the _number_ of their _tracheæ_, but I shall
here describe their whole internal respiratory apparatus. I must
observe that _Reaumur_, _Cuvier_, and most modern writers on the
physiological department of Entomology, have affirmed that they
respire the _water_, and that they receive it for that purpose at
their anal extremity: but M. Sprengel, from having observed in the
larvæ abdominal spiracles, is unwilling to admit this as a fact[290];
and De Geer also seems to hesitate upon it, especially as he
discovered that the animal seemed to absorb the water to aid it in
its _motions_[291]. But when we consider that it is by the action
of a _pneumatic_ apparatus that the absorption and expulsion of the
water takes place, and that the animal when it has been taken out
of that element, upon being restored to it, immediately has _eager_
recourse to this action[292], we shall feel inclined rather to adopt
the opinion of those great physiologists Reaumur, Lyonet, and Cuvier,
and admit that it absorbs water for the purpose of _respiration_.
I shall now explain how this takes place. The pieces both internal
and external that close the anal orifice have been before described;
the others employed in the admission and expulsion of the water are
evidently _respiratory_ organs. When this orifice is opened, the
parts that are above it are drawn back in an opposite direction, so
that the five last segments of the abdomen become entirely empty,
and form a chamber to receive the water that enters by it. When the
water is to be expelled, the whole mass of air-vessels which had
receded towards the trunk, is pushed forwards, and forms a piston
that again expels the water in a jet. It consists of an infinite
number of _bronchiæ_, entangled with each other, which proceed from
the middle and posterior end of the _tracheæ_. M. Cuvier in the
interior of the _rectum_ of the larva discovered twelve longitudinal
rows of little black spots, in pairs, which exhibited the resemblance
of six pinnated leaves. These are minute conical tubes, of the spiral
structure of _tracheæ_, which decompose the water, and absorb the
air contained in it. He also discovered that each of these tubes
gave birth to another outside the _rectum_, which connected itself
with one of the six great longitudinal _tracheæ_; two of which are
of enormous size, and appear to serve as reservoirs, since they
furnish air by transverse branches to two other tubes; they have each
a recurrent branch, which follows the course of the intestinal canal,
and furnishes it with an infinity of _bronchiæ_[293]. These _tracheæ_
are found in the perfect insect. The principal ones in some send
forth many branches, terminating in vesicles, which in shape resemble
the seed-vessels of some species of _Thlaspi_, while others appear to
form a file of oblong ones[294]. Near each of their spiracles also is
a vesicle which appears to be a reservoir[295].
But this kind of structure is not confined to insects strictly
_aquatic_. Even such species of _terrestrial_ ones as live upon aquatic
plants, and are, consequently, necessarily or accidentally often a
considerable time under water, are furnished with some apparatus by
means of which they can exist in this element for a considerable
period. For example, most of the Weevils (_Rhyncophora_) die in a short
time if immersed in water; yet the species of the genera _Tanysphyrus_,
_Bagous_, and _Ceutorhynchus_ which feed on aquatic plants, can
exist for days under water, as I have ascertained by experiment. _C.
leucogaster_ and another of the same tribe, swims like a _Hydrophilus_,
and will live a long time in a bottle filled with water and corked
tight. Other insects also, that are not at all aquatic, have pneumatic
pouches. A striated or channeled vesicle I have found under the lateral
angles of the _collar_ in the humble-bee, where Chabrier supposes the
vocal spiracles are situate; and also at the mouth of the spiracles
of the _metathorax_ in _Vespa_, &c.[296] In _Sphinx Ligustri_ the
_bronchiæ_ terminate in oblong vesiculoso-cellular bodies, almost
like lungs[297]; in _Smerinthus Tiliæ_ these are preceded by a simple
vesicle bound with spiral fibres[298]. M. Chabrier thinks that these
air-bladders of insects, amongst other functions, give more fixity and
force to the muscles for flight[299].
Many physiologists have seen an analogy between the _spiral_ vessels
of _plants_ and the _tracheæ_ of _insects_; and some of great name,
as Comparetti, Decandolle, and Kieser, have thought that in some
instances they terminated in the _oscula_ or cortical pores: but
Sprengel contends that they are not accurate in this opinion[300]. In
fact, the principal analogy seems to be in the _spiral_ structure of
both these vessels.
* * * * *