Part 4
2. With regard to their _shape_, spiracles vary considerably. In general we may observe that the abdominal ones are usually flat, while those of the trunk are often convex[172]. Sometimes they are very narrow and nearly linear, as in many pupæ of _Lepidoptera_, and those in the _metathorax_ of the sand-wasps (_Ammophila_) and affinities; at others they are wider and nearly elliptical, as in _Lucanus_ and many Lamellicorn beetles: again, in _Copris_ they are circular; in _Cordylia Palmarum_ ovate; in _Dytiscus_ oblong[173]; in _Goerius olens_ lunulate; in _Gonyleptes_ nearly of the shape of a horse-shoe[174]; and probably many other forms might be traced, if a thorough investigation with this view were undertaken.
3. The _colour_ of spiracles will not detain us long. In the caterpillars of _Lepidoptera_ this is often so contrasted with that of the rest of the body, as to produce a striking and pleasing effect. Thus when the body is of a _dark_ colour, they are usually of a _pale_ one[175]; or if the body is _pale_, they are _dark_[176], or surrounded with a dark ring[177]. This contrast is often rendered more striking by their position with regard to the partial colours that often ornament caterpillars: in those whose sides are decorated by a longitudinal stripe, the spiracles are often planted in it[178]; or just above it[179]; or between two[180]: in some hawkmoths the intermediate ones are set in white or pale spots, which gives great life to the appearance of the animal. In general, in perfect insects the most prevalent colour is buff, or reddish-yellow. In the larva of the great water-beetle these organs resemble the iris of the eye, being circular with concentric rings alternately pale and dark[181].
4. The _size_ of spiracles varies considerably. Those in the larva 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.