last mentioned are so minute as to be scarcely visible except under a
lens, while those behind the fore-legs in the mole-cricket are a full
line in length, and those in the _pleura_ of _Acrocinus accentifer_, a
Brazilian Capricorn beetle, are more than twice as long. In the same
species they are often found of different sizes;--thus the _anal_ pairs
in the water-beetle lately alluded to, I mean in the perfect insect,
are much larger than the rest[182], probably that the animal may imbibe
a larger quantity of air when it rises to the surface of the water,
where it suspends itself by the _tail_. In those Lamellicorn beetles in
which the terminal part of the abdomen is not protected by the elytra,
the _covered_ spiracles are the largest.
5. Under the next head, the _situation_ of spiracles, I shall not
only consider the part of the body in which they are situated, but
likewise their _position_ in the crust; to which last, as it will not
detain us long, I shall first call your attention. Their position in
this respect is most commonly _oblique_: but in the abdomen of the
above water-beetle they are _transverse_, and in a larva I possess,
probably of an _Elater_, they are _longitudinal_. In spinose
caterpillars these organs are generally planted between two spines,
one being above and the other below. The _lateral_ line of the body
most commonly marks their situation; but in many cases they become
_ventral_, and in others _dorsal_. The most important circumstance,
however, connected with the present head is their appropriation to
particular segments or parts of the body, for, like the ganglions of
the spinal marrow, they are distributed to almost every segment. Let
us take a summary view of their arrangement in this respect.
No insect has any spiracle in the _head_; but in caterpillars and many
other _larvæ_ there is a pair in the _first_ segment of the trunk. This
is also to be found in the other states, but is not easily detected in
the _pupæ_ of _Lepidoptera_: in the _Coleoptera_ order, in the grub
of the Lamellicorn beetles, it is extremely conspicuous, and planted
in the side of the first segment[183]; in other Coleopterous grubs
it is not so readily found, but probably its station is somewhere
behind the base of the arms, where it is very visible in that of
the _Staphylinidæ_. In the _imago_ of insects of this order, this
antepectoral spiracle has been overlooked, and indeed is not soon
discovered: to see it clearly, the manitrunk should be separated from
the alitrunk; and then if you examine the _lower_ side of the cavity,
you will see a pair of, usually, large spiracles planted just above
the arms, in the ligament that unites these two parts of the trunk to
each other: in the common rove-beetle, however, (_Goerius olens_)you
may easily see it without dissection[184]. In the _Orthoptera_ it is
situated behind the arms, as in _Gryllotalpa_: or between them and
the _prothorax_, as in _Blatta_: in the _Hemiptera_ and _Neuroptera_
probably the situation is not very different. In the _Lepidoptera_
this pair of spiracles is planted just before the base of the upper or
primary wings[185]: a similar situation, I suspect, is appropriated to
it in the _Trichoptera_, but covered by a tubercle or scale. Something
similar has been noticed by M. Chabrier, in the same situation and
circumstances, in the collar of _Hymenoptera_[186]. In numerous
_Diptera_ this breathing pore is planted on each side between the
collar and the _dorsolum_ above the arms[187], and in _Hippobosca_ in
the collar itself[188].
In _Lepidopterous_, _Coleopterous_, and some other larvæ, the
two segments of the body corresponding with the alitrunk in the
perfect insect, are without spiracles, neither have they in this
state, though pneumatic organs have been discovered[189], any real
ones in that part: but not so the _remaining_ orders, all of which
have these organs in that section of the trunk. To begin with the
_Orthoptera_:--in _Blatta_ there seems to be a long narrow one
behind the intermediate leg; in the _Gryllotalpa_ there is one in
the posterior part of the _pleura_; and in _Locusta_, above both the
intermediate and hind legs[190]. It is probable, that in general
those that have _no_ spiracles in the manitrunk have _four_ in the
alitrunk, which seems the natural number belonging to the trunk.
In many of the Heteropterous _Hemiptera_ in the _parapleura_
there is an open spiracle without lips[191], to which, as in that
beautiful bug _Scutellera Stockeri_, a channel sometimes leads. The
space in which this spiracle is planted in other genera of bugs
(_Pentatoma_ &c.) is covered with a kind of membranous skin, often
much corrugated[192]. In the aquatic insects of this section, and
many terrestrial ones, as _Reduvius_, &c. this spiracle is obsolete.
There is another circumstance, possibly connected with their
respiration, relating to many of the bugs, which may be mentioned
here. If you examine _Pentatoma rufipes_, a very common one, you will
find between the _scapula_ and _parapleura_ a long orifice or chink;
this upon a closer inspection, under a good magnifier, you will see
completely filled with minute stiff hairs or bristles, which fringe
the posterior margin of the _scapula_[193]. In a Brazilian species of
_Lygæus_ (_sexmaculatus_ K. M. S.) with incrassated posterior thighs,
these hairs are replaced by lamellæ which have the aspect of _gills_.
A red, vertical, convex spiracle, with its orifice towards the head,
and terminating posteriorly in a kind of conical sac, is situated
towards the hinder part of the _pleura_ in the giant water-scorpion
(_Belostoma grandis_[194]); this seems analogous to one lately
mentioned in the mole cricket. In the other section of this Order it
is not easy to decipher the parts of the under side of the alitrunk.
In _Fulgora_, _Cicada_, and many others of its genera, there appears
to be more than one opening into the chest; but whether they are of a
pneumatic nature or not, can only be ascertained by an inspection of
the living animal. There is a very visible spiracle over each of the
four last legs of the _Libellulina_[195], but in the remainder of the
_Neuroptera_ Order they have eluded my search. In the _Hymenoptera_
and _Diptera_ they are nearly in the same situation, being placed
behind the wings on each side of the _metathorax_; in the latter
Order with the poiser near them on the inner side[196]: in this also,
the spiracles of the _trunk_ are without _lips_, except in the larvæ,
but are often merely an orifice, sometimes fringed with hairs; this
is particularly conspicuous in _Syrphus_, in which these orifices are
very large, and in some species closed by an elegant double fringe
of white hairs. This is doubtless to prevent the entrance of any
particles of dust or the like.
We are next to consider the situation of the spiracles of the
_abdomen_: these which are supposed to be appropriated exclusively
to _inspiration_, are usually more numerous than those of the
trunk, by which it is probable that _expiration_ is performed, and
have principally attracted the notice of Entomologists: they are
either dorsal, lateral, or ventral. In _Dytiscus_, _Copris_, &c.
amongst the beetles, all the spiracles are _dorsal_; in the larvæ
of _Coleoptera_ and _Lepidoptera_ they are _lateral_; and in the
Heteropterous _Hemiptera_ they are usually _ventral_: in _Dynastes_
they are commonly found of all three descriptions;--the _three_
first being _dorsal_, the _two_ next _lateral_, and the _last_
pair _ventral_[197]. In some instances, as in _Perga Kirbii_, and
probably other _Hymenoptera_, these organs are planted in that
portion of the dorsal segment which turns under, as was observed
in a former letter[198], and becomes ventral. Generally there is
a _pair_ of spiracles to _each_ segment, and in those insects that
have a hypochondriack joint[199] there is often a spiracle in it.
The last segment of the abdomen is always without these orifices,
as is the basal one in _Velia_, _Ranatra_, and some other bugs. A
singular anomaly distinguishes the _Libellulina_: they appear to
have no _abdominal_ spiracles[200], yet I have seen the abdomen of
_Libellula depressa_ when reposing, contract and dilate alternately,
from whence it follows that this part is concerned in respiration.
Sprengel says that the larvæ in this tribe have seven or nine on
each side[201], and Reaumur speaks of them as discoverable in the
pupa[202]. I have carefully examined the pupa-skin of most of the
genera of _Libellulina_, under a powerful magnifier, but have not
succeeded in discovering any thing like these organs in the abdomen.
The _Ephemera_ and probably the other _Neuroptera_ have abdominal
spiracles[203]. M. Latreille observed one on each side of the base
of the scale on the footstalk of the abdomen in ants[204]. Generally
the abdominal spiracles may be described as planted in the _crust_
of the insect; but in many cases their station is in the membranous
folds, which I have therefore named the _pulmonarium_, that sometimes
separate the dorsal from the ventral segments: these folds allow
of a considerable distention of the abdomen, which is probably
necessary when all the air-vessels are full. In a gravid _Ichneumon_
I once saw it enlarged to more than twice its natural size by
means of this membrane, through which the eggs were distinctly
visible.--Before I bid adieu to this subject, I must say a few words
upon the situation of the organs in question in the _myriapods_. In
_Iulus_, in each segment is a pair of orifices which have usually
been regarded as spiracles, but M. Savi found that these orifices
opened into vesicles containing a fetid fluid, and upon a very close
examination he discovered the real spiracles above the base of the
legs, in connexion with _tracheæ_[205]. In some of the larger species
of _Scolopendræ_ large open spiracles in the same situation are
extremely visible[206]. _Cermatia_ presents a singular anomaly:--a
single series of spiracles of the usual form, each planted in a cleft
of the posterior margin of the dorsal _scuta_, runs along the back
of the animal[207]: unless we may suppose that, like the seeming
spiracles of _Iulus_ just mentioned, these are merely orifices by
which it covers itself with some secretion.
6. A few words upon the _number_ of spiracles.--If you examine the
common dog-tick (_Ixodes Ricinus_), you will find only _one_ of these
organs on each side of the abdomen[208]; the _Libellulina_, as we
have seen, have only _four_, all in the trunk; in the _Dynastidæ_,
_Melolontha_, and the larva of _Dytiscus_, there are _fourteen_;
_sixteen_ in the _Copridæ_; _eighteen_ in _Dytiscus_, and probably the
majority of _Coleoptera_, both larva and imago, and _Lepidoptera_; and
a pair to each segment except the last, in the _Myriapods_.
ii. _Respiratory plates_ (_Respiratoria_). The nearest approach to
spiracles is made by those remarkable plates that are found in such
larvæ of _Diptera_, as in that state inhabit substances that might
impede or altogether stop the entrance or exit of the air by the
ordinary spiracles, such as dead or living flesh, dung, or the like.
The CREATOR therefore, as he has seen it good for wise reasons[209] to
commission certain insects to feed on unclean food, has fitted them for
the offices that devolve upon them, and has placed their orifices for
breathing in plates at each extremity of the body. There are usually
two of these plates at the head, and two at the tail. In the grub of
the common flesh-fly (_Sarcophaga carnaria_), at the junction of the
first segment of the body with the second, two of these plates are
planted, which are concave and circular, with a denticulated margin; in
the cavity near the lower side is a round spiracle. These plates the
animal can withdraw within the body, so as to prevent this spiracle
from being stopped up by any greasy substance[210]. The posterior
extremity of this grub is truncated, and has a large and deep cavity
surrounded by several fleshy prominences: at the bottom of this are
two oval brown plates, in each of which are _three_ oval spiracles,
placed obliquely: by the contraction of the fleshy prominences, this
cavity also can be closed at the will of the animal[211]. In some
cases, several stiff rays or spines replace the prominences[212].
In _Echinomyia grossa_ and others the anal plates appear not to be
perforated, being surmounted only by a central boss[213]; but this,
most probably, as in the case of _Œstrus Ovis_[214], is a _valve_
that closes the respiratory orifices. In the gad-fly of the ox (_Œ.
Bovis_) there are no plates at the _anterior_ extremity of the body;
but those planted in the _other_ end are very remarkable, and demand
particular attention. Each is separated by a curved line into two
unequal portions; the smallest of which is contiguous to the convex
belly, and the largest to the concave back of the animal. This
last is distinguished by two hard, brown, kidney-shaped pieces, a
little elevated with the concave sides turned towards each other: in
this sinus is a _single_, small, white spot, which appears to be a
spiracle: in the smallest portion are _eight_ minute circular orifices,
arranged in a line[215]. As the only communication which this grub has
with the atmosphere is at its _anal_ extremity, it has no occasion
for respiratory organs at the _other_. The gad-fly of the horse
(_Gasterophilus Equi_, &c.) which has no communication at all with the
external air, breathing that which is received into the stomach, has
these plates at _both_ ends of the body.
iii. _Respiratory Appendages_[216]. These may be divided into _two_
kinds; those by which the animal has _immediate_ communication with
the _atmosphere_, and those by which it extracts _air_ from _water_.
1. To begin with the _first_. These are often found in insects
which, during their two first states, live in the water. No better
example, nor one more easy to be examined, of this structure, can
be selected, than the gnat (_Culex_). You must have occasionally
observed in tubs of rain-water, numerous little wriggling worm-like
animals, which frequently ascend to the surface; there remain a
while, and then bending their head under the body rapidly sink to
the bottom again. 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 its body with a
singular organ, which varies in length according to the species, and
forms an angle with the last segment but one[217]. The mouth of this
organ is tunnel-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 the mouth when the animal is disposed to sink in
the water, are used to close it, and 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 that element, 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. Its anal
extremity is clothed with bunches of hairs, which are furnished with
some repellent material which prevents their becoming wet[218]: 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[219].
When the gnat undergoes its first change and assumes the pupa,
instead of a _single_ respiratory appendage it is furnished with
a _pair_, each in shape resembling a cornucopia, and, what is
remarkable, placed near the opposite extremity of the body, for they
proceed from the upper side of the trunk[220]. By these tubular
horns, which Reaumur compares to asses' ears[221], they respire, and
are suspended at the surface.
Other respiratory tubes or horns are more complex. The rat-tailed grub
of a fly (_Helophilus pendulus_), like the gnat, breathes by a tube:
but as if the CREATOR willed to show those whose delight it is to
investigate his works, by how many varying processes he can accomplish
the same end, this respiratory organ is of a construction totally
different from that we have been considering. It is not fixed to the
side of the tail, but is a continuation of the tail itself, and is
composed of two tubes, the inner one, like the tube of a telescope,
being retractile within the other[222]. The extremity, which is very
slender, and through which the air finds admission by a pair of
spiracles, terminates in five diverging hairs or rays, which probably
maintain it _in equilibrio_ at its station at the surface[223]. As
these larvæ seek their food amongst the mud at the bottom of shallow
pools, in which they are constantly employed, they require an apparatus
capable of being lengthened or shortened, to suit the depth of the
water, that they may maintain their necessary communication with the
atmosphere; and for this purpose a _single_ tube would not have been
sufficient: therefore PROVIDENCE has furnished them with _two_, and
both are extremely elastic, consisting of annular fibres, so as to
admit their being stretched to an extraordinary length. Reaumur found
that these animals could extend their tails to near _twelve_ times
their own length. The mechanism by which the terminal piece is pushed
forth or retracted, is very curious, though extremely simple. Two large
parallel _tracheæ_, the direction of which is from the head[224] of
the grub to its tail, occupy a considerable portion of its interior:
near the origin of the tail, where they are very ample, they suddenly
grow very small, so as to form a pair of very slender tubes, but so
long that, in order to find room in a very contracted space, they form
numerous zigzag folds attached to the terminal tube; when this issues
from the outer tube they consequently begin to unfold, and when it is
intirely disengaged, they are become quite straight and parallel to
each other. Reaumur has figured them as being united at the _base_ of
the inner tube[225]; most probably, however, they do not here stop
short, but, as in other instances, proceed to the end, and terminate
in the two spiracles mentioned above: he conjectures that when the
animal has occasion to push forth its respiratory apparatus, it injects
into these vessels part of the air contained in the body of the
_tracheæ_, which of course would cause them to unfold and push forth
the tube[226]. When this insect assumes the pupa, instead of its anal
respiratory organ it has _four_ respiratory horns in the trunk near the
head[227].
The larva of the chamæleon-fly (_Stratyomis Chamæleon_) is furnished
with a respiratory organ of a still different and more elegant
structure, exhibiting some resemblance to the _tentacula_ of what
are called sea anemones. In this larva the last joint of the body is
extremely long, and terminates in an orifice to receive the air, which
is surrounded by a circle of about thirty diverging rays, consisting
of beautifully feathered hairs or plumes[228]. This apparatus serves
the same purpose with that above described of the larva of the gnat.
The feathery hairs are so prepared as to repel the water, and thus to
suspend the animal by its tail at the surface, and preserve a constant
access of air. When it has occasion to sink, it turns these hairs in
and shuts the orifice, carrying down with it an air-bubble that shines
like quicksilver, and which Swammerdam conjectures enables it again to
become buoyant when it wants to breathe[229].
In the red aquatic larva of a small gnat (_Chironomus plumosus_)
there are _two_ anal respiratory subcylindrical horns, with the
orifice fringed with hairs[230]; and in another gnat Reaumur
discovered _four_[231]. The larva of _Tanypus maculatus_, whose
remarkable _legs_ I formerly noticed[232], exhibits in the _interior_
of its trunk two long, oval, opaque bodies, which De Geer conjectures
may be air-reservoirs; these, when the animal assumes the pupa,
according to every appearance become _external_, and are placed on
the back, precisely where the respiratory horns of aquatic pupæ
are usually situated,--they appear to terminate in a transparent
point[233]. The pupa of a _Tipula_ observed by Reaumur, instead of
_two_ has only _one_ of these respiratory organs, in the form of a
very fine hair proceeding from the anterior end of the trunk, and
considerably longer than the animal itself[234].
It is observable that aquatic insects that come to the surface of the
water for air, receive it at the anus, often carrying it down with
them as a brilliant bubble of quicksilver. This is generally done
by means of spiracles in perfect insects, but in the water-scorpion
tribe in that state respiration is by means of a long hollow tube,
consisting of two concavo-convex pieces which apply exactly to each
other. This is found in both sexes, and therefore cannot be an
_ovipositor_, as some have thought[235].
These respiratory organs, however, are not invariably confined to
_aquatic_ larvæ and pupæ, for those of some aphidivorous flies have
anal ones, and the pupa of _Dolichopus nobilitatus_, or a fly nearly
related to it, which is _terrestrial_, has likewise a pair of long
sigmoidal ones on the back of the trunk[236]. The pupa also of the
rat-tailed larva just noticed as having _four_ horns, resides under
the _earth_, the insect being only _aquatic_ in its grub state.
2. I am next to consider those respiratory appendages by which aquatic
insects, since they do not come to the surface for that purpose, appear
to extract air for respiration from the _water_; so that they may be
looked upon in some degree as analogous to the _gills_ of fishes:
there is, however, this difference between them--in fishes, the blood
is conveyed in minute ramifications of the arteries to the surface of
the branchial laminæ, through the membranes of which they abstract the
air combined with the water; but as insects have no circulation, the
process in them must be different, and their branchiform appendages may
be regarded as presenting some _analogy_ rather than any _affinity_
to those of fishes. The first approach to this structure is exhibited
by the pupa of a gnat lately mentioned (_Chironomus plumosus_); for
on each side of the trunk this animal has a pencil consisting of five
hairs elegantly feathered, which, when they diverge, form a beautiful
star; its anus also is furnished with a fan-shaped pencil of diverging
hairs[237].
On most of the abdominal segments of the larvæ and pupæ of the
_Trichoptera_ are a number of white membranous floating threads,
arranged in bundles, _four_ on each segment, two above and two below,
and traversed longitudinally by several air-vessels or _bronchiæ_,
which run in a serpentine direction, growing more slender as they
approach the extremity, and in some places sending forth very
fine ramifications,--these are their respiratory organs[238]. The
caterpillar also of a little aquatic moth (_Hydrocampa stratiotata_)
at first sight appears to be covered on each side with hairs, but
which examined under a microscope are found to be branching flattish
filaments, each furnished with tubes from the _tracheæ_. These
caterpillars have also the semblance of spiracles, but apparently
found in the usual situation[239]. The larva of a little beetle often
mentioned in my letters (_Gyrinus Natator_), is furnished on each
side of every abdominal segment with a long, hairy, slender, acute,
conical process, of the substance of the segment, through each of
which an air-tube meanders; the last segment but one has _four_ of
these processes, longer than the rest[240].