Before I call your attention to what I would denominate an imperfect mouth, in which some one or more of the seven organs above enumerated exist under another form, or only as rudiments,—I must say something upon the mouth of the Myriapods and Arachnida, in which there seem to be redundant organs of manducation.—M. Latreille, in the Essay lately quoted, in which, though some of his notions seem fanciful, he has shown a vast depth and range of thought and research, has asserted,—from the admirable and curious observations of M. Savigny, and those which since their publication he has made himself,—that the masticating organs of annulose animals (called by him condylopes) are a kind of legs[1345]. And M. Savigny, whose indefatigable labours and unparalleled acuteness have opened the door to a new and vast field in what may be denominated analogical anatomy,—has observed, that with certain Apiropods[1346] the organs that serve for manducation do not differ essentially from those which, with the other Apiropods and the Hexapods, serve for locomotion[1347]: and the unguiform mandibles of the larvæ of certain Diptera, you have before been told, are used not only in manducation, but also as legs[1348]. These remarks will satisfactorily prove to you, that organs which at first sight possess no visible affinity or analogy—as for instance, jaws and legs—may, if traced through a long series of beings, exhibit a very great one;—and will lessen your surprise when you find, that in certain tribes such commutations of organs and their use take place.
The following is the structure, as to its organs, of the mouth of the myriapods, as exhibited by the centipedes (Scolopendridæ). The part which appears to perform the office of the upper lip (but which M. Savigny regards as the nose, calling it the chaperon,) is a transverse piece with a deep anterior sinus, in the centre of which is a minute tooth[1349]. This piece is separated from the forepart of the head by a suture; but it probably is not moveable: however, it covers the mouth, and may be regarded rather as analogous to the labrum. Below this are two mandibles, armed at their end with five sharp triangular teeth[1350], under which are the maxillæ, terminating in a moveable concavo-convex lobe, resembling the valve of a bivalve shell[1351]; and between them is the labium, of a rhomboidal shape, divisible into two lobes, attached laterally to the maxillæ: these lobes M. Savigny terms the second maxillæ, forming with the others, according to him, the labium[1352]. Affixed to the base of this labium, or covering it on the outside, are a pair of pediform palpi, which he considers as the first auxiliary labium, and representative of the first pair of legs of hexapods and Iuli[1353]. I imagine them to be also the analogues, in some degree, of the labial palpi of a perfect mouth. The last of the organs in question is a large rhomboidal plate affixed to the first apparent segment of the trunk, crowned at its vertex with two truncated denticulated teeth, and from the upper sides of which emerge a pair of moveable organs terminating in a powerful incurved claw, and which entirely covers all the other parts of the mouth[1354]. This, M. Savigny deems as a second auxiliary labium, and the lateral organs of prehension,—which may be regarded each as a kind of maxillary hand, and as the only representatives in this tribe of the maxillary palpi, though widely different,—he looks upon as really analogous to the second pair of legs in Iulus and the hexapods[1355]. These two pairs of pedipalpes (to use an expressive French term) show their relation to legs by their general structure, and their analogy with palpi by their use as oral organs, though belonging to the trunk: so that here we see the legs and their appendages assume a material function in manducation, forming a singular contrast to what we had observed before with regard to mandibles becoming instruments of locomotion. The mouth of the Iulidæ, with little variation, is upon the same plan[1356] with those here described.
The next type of form with regard to the oral organs is that of the Arachnida. In these, as you know, the head is confounded with the trunk; so that they are a kind of Blemmyes in the insect world. Their organs of manducation, amongst which there is no labrum or upper lip, are, in the first place, a pair of mandibles planted close and parallel to each other in the anterior part of the head, which they terminate. In the spiders they consist of two tubular joints, of which the first is much the largest, more or less conical or cylindrical, and armed underneath with a double row of stout teeth; and the terminal one is more solid and harder, in the form of a very sharp crooked claw, which in inaction is folded on the first joint between the teeth. Under its extremity on the outside is a minute orifice, destined to transmit a venomous fluid, which is conducted there by an internal canal from the base of the first joint, where is the poison-bag[1357]. In the scorpion and harvest-man (Phalangium) the mandible consists of two joints terminated by a chela or double claw, the exterior one being moveable[1358].—M. Latreille, as has been before observed, regards these not as representatives of the mandibles of hexapods, but as replacing the interior pair of antennæ, in the situation of which they are precisely placed, of the Crustacea[1359]: and M. Savigny is of opinion that the Arachnida may in some sort be defined as Crustacea without a head, and with twelve legs, of which the two first pair are converted into mandibles and maxillæ[1360]. From the situation of the organs in question, the first of these opinions seems preferable; but the conversion of the legs in other cases, at least the coxæ, into organs of manducation, gives some weight to the last. With regard to their use, it is said to be to retain the insect which the animal has seized, and to facilitate the compression which the maxillæ exercise upon it for the extraction of the nutritive matter[1361]. If this be correct, in this respect the mandibles may be said to represent the maxillæ of the mandibulate hexapods; and, vice versa, the sciatic maxillæ, as they have been denominated[1362], of the Arachnida, their mandibles. The palpi are pediform, and the first joint of the coxa, or hip, acts the part of a maxilla:—this is composed of a single piece or plate, more or less oval or triangular, sometimes straight and sometimes inclined to the labium, with the interior extremity very hairy. The labium consists also of a single piece, and is only an appendage of the anterior extremity of the breast. The interior of the mouth, or palate, presents a fleshy, hairy, linguiform piece, which is usually applied to the internal face of the labium. An opening is supposed to exist in its sides, for the transmission of the alimentary juices[1363]. If you examine the under side of the body of a scorpion, you will find that not only the palpi, but the two anterior pair of legs, by means of their coxæ, are concerned in manducation: so that these insects have in fact three pairs of maxillæ—a circumstance that M. Savigny has observed to take place also in the harvest-men (Phalangium L.)[1364]. The palpi of the scorpion, which may be called its hands, like the anterior legs of the lobster and crab, terminate in a tremendous chela or forceps, consisting of a large triangular joint, armed at the end with a double claw internally toothed; the exterior one of which, contrary to what takes place in the animals just named, is moveable, and not the interior[1365].
Having given you this full account of the trophi of those animals that have all the organs of manducation developed, I must next advert to those in which one part receives an increment at the expense of others, and the whole oral machine is fitted for suction; or where some parts appear to be deficient, so that this may be called an imperfect mouth. At first sight one would regard the trophi of a bee as of this description; but this is not the case, since it has all the ordinary organs, though the tongue is unusually long, and looks as if it was made for suction; which, however, as you have been informed, is not the case.
There are five kinds of imperfect mouth to be met with in insects that take their food by suction, each of which I shall distinguish by a separate denomination. The first is that of the Hemiptera Order:—this I term the Promuscis; the second is that of the Diptera, which with Linné I call Proboscis; the third, peculiar to the Lepidoptera, is with me an Antlia; the fourth, which I name Rostrulum, is confined to the Aphaniptera order, or genus Pulex L.; and the last is Rostellum, which I employ to denote the suctory organs of the louse tribe (Pediculidæ).
i. Promuscis[1366].—The organ we are first to consider has usually been denominated Rostrum: but since that term is likewise in general use for the snout of insects of the weevil tribes (Curculio L.), I think you will concur with me in adopting the one here proposed, for the very different oral instruments of the Hemiptera. Illiger has employed promuscis to denote those of bees[1367]: but since, as I have just observed, they consist of all the ordinary organs, they seem to require no separate denomination: the term, therefore, may be applied to represent a different set of trophi, without any risk of producing confusion. This part consists of five pieces: viz. a minute, long, conical piece, commonly very slender, which covers the base of the promuscis, and represents the labrum[1368]; a jointed sheath (vagina), consisting of either three or four joints, the analogue of the labium, and four slender rigid lancets (scalpella), the two exterior ones, according to M. Savigny, representing the mandibles, and the intermediate pair the maxillæ[1369]. By the union of these four pieces a suctorious tube is formed, which the animal inserts into the substance, whether animal or vegetable, the juices of which form its nutriment. These pieces are dilated at their base, and serrated at their apex; and the two central ones, though at their origin they are asunder, form one tube, which has often been mistaken for a single piece. A pharynx and tongue have been discovered by M. Savigny in this apparatus; who thinks that in Nepa there are also rudiments, but very indistinct, of labial palpi: so that the maxillary palpi seem to be the only part absolutely wanting[1370].
The Promuscis when at rest is usually laid between the legs; but when employed, in most cases its direction is outward. In the genus Chermes L. (Psylla Latr.) the origin of the promuscis has been supposed to be in the breast; but if closely examined, this anomaly in nature will be found not to exist. If you take one of these insects, the first thing that strikes you upon inspecting the head, is a pair of remarkable conical processes into which the front appears to be divided. Look below these, and you will there discover the upper-lip: and from this you may follow the promuscis till it gets beyond the forelegs, when it takes a direction perpendicular to the body[1371]; a circumstance which has given rise to the above false notion. Though in Coccus, Chermes, &c. this instrument is short, in some Aphides it is longer in proportion than in any other insect. In A. Quercus it is three times the length of the body; so that when folded, it stretches out beyond it, and looks like a long tail[1372]; and in A. Abietis it even exceeds that length[1373].
ii. Proboscis[1374].—Linné long since, and after him Fabricius, has employed this term to designate the oral instruments, or rather their sheath, in the Muscidæ and some others, calling the same organ, when without fleshy lips, rostrum and haustellum: but as the parts of the mouth in all true Diptera (for Hippobosca and its affinities can scarcely be deemed as co-ordinate with the rest), are analogous to each other; although in some they are stiff and rigid, in others flexile and soft, and in Œstrus (except the palpi) mere rudiments,—the same appellation ought to designate them all. I am happy to find that M. Latreille agrees with me in this opinion; and to his sensible observations on this head, if you wish for further information, I refer you[1375]. The mouth of Dipterous insects appears to vary in the number of pieces that it presents; but in all, the theca or sheath is present, which represents the labium (including the mentum) of the mandibulate Orders[1376]. It consists of three joints, the last of which is formed by the liplets (Labella). Those in the Muscidæ are large, turgid, vesiculose, and capable of dilatation; in the Bombylidæ and other tribes they are small, slender, long and leathery, and sometimes recurved. The second joint or stalk, which may be said to represent the mentum, the liplets being properly in a restricted sense the analogue of the labium, its sides being turned up, forms a longitudinal cavity, which contains the haustellum. The upper piece of this, the valvula, is long, rigid, and very sharp, representing the labrum[1377]. Beneath this cover, in the above cavity, are the lancets; which, as far as they are at present known, vary in number and form: sometimes there are five of them, sometimes four, sometimes two, and sometimes, it should seem, only one[1378]. In the gnat (Culex) they are finer than a hair, very sharp, and barbed occasionally on one side[1379]; in the horse-fly (Tabanus L.) they are flat and sharp like the blade of a knife or lancet[1380]. In this tribe the upper pair, or the knives (Cultelli), represent the mandibles; the lower pair, or the lancets (Scalpella), usually palpigerous, the maxillæ; and the central one the tongue. In the horse-fly Reaumur has figured only four, exclusive of the labrum and labium; but in a specimen I have preserved there appear to be five, one of which, as slender as a hair, I regard as the analogue of the tongue[1381].—When the lancets are reduced to two, they probably represent the maxillæ, the mandibles being absorbed in the labrum; and where there is only one, the maxillæ also are absorbed by the labium, which then bears the palpi, the lancet representing the tongue[1382]. The lancets are so constructed in many cases, as to be able by their union to form a tube proper for suction, or rather for forcing the fluid by the pressure of the lower parts to the pharynx[1383]. Labial palpi appear not usually present in the proboscis; but M. Savigny thinks he has discovered vestiges of them in Tabanus[1384]. In this genus the maxillary ones are large, and consist of two joints[1385]. The proboscis is often so folded, as to form two elbows; the base forming an angle with the stalk, and the latter with the lips, so as in shape to represent the letter Z, only that the upper angle points to the breast, and the lower one to the mouth: this is the case with the flesh-fly and many others. In other flies, as Conops and Stomoxys, whose punctures on our legs so torment us[1386], there is only a single fold, with its angle to the breast. The proboscis is received in a large oblong cavity of the underside of the anterior part of the head.