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.