With respect to its substance, the labium in most Coleopterous insects is hard and horny, in Necrophorus it is membranous, and the mentum harder; in Prionus coriarius, our largest Capricorn-beetle, both are membranous; in the bee-tribes, Apis L., the labium rather resembles leather, while the mentum is hard. Its surface is often convex, sometimes plane, and sometimes even concave; as for instance in Melolontha Fullo, a rare chafer occasionally found on the coast of Kent. In some it is covered with excavated points; in others it is quite smooth. In numbers, as in the Predaceous beetles, both labium and mentum are perfectly naked; in others, as in the common cockchafer, they are hairy; in Geniates barbatus Kirby, another chafer in the male insect, the labium is naked, while the mentum, which forms a piece distinct from that part, is covered with a dense rigid beard[1200]. In shape the whole labium varies considerably, much more than the labrum; for in addition to most of the forms I enumerated when I described that organ, which I shall not here repeat, you may meet with examples of many others. Thus, to instance in the Petalocerous tribes (Scarabæus L.), in some, as in the Rutelidæ, the labium is urceolate, or representing in some degree the shape of a pitcher[1201]; in others it is deeply concave, and not a little resembles a basin or a bowl[1202]; this form is peculiar to the labium of Cremastocheilus Knoch, a scarce North American beetle; in another related to this, but of an African type (Genuchus Kirby MS. Cetonia cruenta F.), it is a trapezoid plate, which is elevated from the head, and hangs over the throat like a chin[1203]. In the Hymenoptera it is extremely narrow and long, and embraces the sides of the tongue, as well as covering it from below; so that it wears the appearance of a kind of tube[1204]. Generally speaking, the length of the labium exceeds its breadth; but in the Predaceous beetles the reverse of this takes place, it being very short and wide, and usually terminating towards the tongue in three lobes or teeth which form two sinuses varying in depth[1205].
The mentum taken by itself affords no very striking characters to which I need call your attention: I shall only observe, that in Hymenoptera it is generally of a triangular shape[1206]; but before I proceed to consider the labial palpi, it will be proper to notice the remarkable labium of Orthopterous insects, and of the Libellulina, between which there is no little analogy. At first you would imagine the terminal part of this organ in the former to be the analogue of the tongue, or ligula F.; as it is indeed generally regarded by modern Entomologists[1207]. It seems, like the tongue of the Carabi L., Dytisci, &c., to be a distinct piece, which has below it both labium and mentum; but when you look within the mouth, you will find a linguiform organ[1208], which evidently acts the part of a tongue, and therefore ought to have the name; and the piece just alluded to must either be regarded as the termination of the lip, or as an external accompaniment of the tongue, analogous, it may be, to the paraglossæ in bees[1209]. In a specimen of Acrida viridissima (Locusta F.) which I dissected, the tongue is as long as the appendage of the under-lip, and by its upper surface seems to apply closely to it. In the Libellulina a similar organ is discoverable[1210], which on its upper-side terminates in the pharynx, like that of one of the Harpalidæ before mentioned. In the Orthoptera, therefore, I regard the labium as consisting of three articulations, the upper one divided into two, three, or more lobes[1211]; the intermediate one more directly answering to the labium of other insects, and the basal one or mentum. This organ in the Libellulina is of a different structure: it has only two articulations representing labium and mentum; but the former consists of three parallel pieces, the two exterior ones rising higher than the intermediate one, and at their inner angle having an acute sinus from which the palpi emerge; and the intermediate piece, which is longitudinally channelled, lapping over the inner side of the lateral pieces. From the angle of the covered part of these pieces, a subulate short horizontal horn points inwards towards the tongue, which it must keep from closing with the labium[1212].
3. Palpi Labiales[1213].—The last-mentioned organs, the labial palpi, next claim our attention; but before I advert particularly to them, it will be proper to premise a few words upon palpi, or feelers, in general. These are usually jointed moveable organs, of a corneous or coriaceous substance, attached by ligaments to the labium and maxillæ, and in the Crustacea even to the mandibulæ. Their joints, which are usually more or less obconical, articulate also in each other by ligaments, with perhaps some little mixture of the ball and socket. Their ends, the last joint especially, seem furnished with nervous papillæ which indicate some peculiar sense, of which they are the instrument. What that sense is has not been clearly ascertained, and concerning which I shall enter more into detail in another place. Their motion seems restrained, at least in some, to two directions, towards and from the mouth. They were called palpi or feelers, because the insect has been supposed to use them in exploring substances. There seem to be no organs in the vertebrate animals directly analogous to the palpi of insects and Crustacea, unless, perhaps, the cirri that emerge from the lips of some fishes, as the cod, red mullet, &c. which Linné defines as used in exploring (prætentantes). Whether the vibrissæ, miscalled smellers, of some quadrupeds and birds have any reference to them, I will not venture to affirm; but the feline tribe evidently use their bristles as explorers, and they are planted chiefly in the vicinity of the mouth.
Having made these general remarks, I shall now confine myself to the labial palpi. I call them labial palpi, because that term is in general use, and because in many cases they really do emerge from what I consider as the labium, as in most of the chafers; but they might with equal propriety be denominated lingual palpi, since they sometimes appear to emerge from the tongue as in the stag-beetle (Lucanus Cervus). In some instances, as in the Predaceous beetles, they seem to be common to both labium and tongue, being attached at their base on the upper side to the former, and on the under side to the latter. As to their situation: they emerge from the base of the labium in the locusts (Locusta Leach)[1214]; from its middle in Hister maximus[1215]; from its summit in Amblyterus MacLeay[1216]; and from its lateral margin in Dynastes MacLeay, &c. They consist of from one to four joints; which, I believe, they never exceed. They vary in length; though generally shorter than the maxillary palpi, yet in the ferocious tiger-beetles (Cicindela L.) they equal them in length; and in the hive-bee and humble-bees, and many other bees, they are considerably longer[1217]. The two first joints of these palpi, however, in these bees are different in their structure from the two last, being compressed and flat, or concave; and the two last joints, instead of articulating with the apex of the second, emerge from it below the apex: so that these two first joints seem rather elevators of the palpi than really parts of them[1218]. With respect to the relative proportions of their joints to each other: in some cases the first joint is the longest and thickest, the rest growing gradually shorter and smaller[1219]; in others, the second is the longest[1220]; in others, again, the third[1221], and sometimes the last[1222]; and often all are nearly of the same size and length[1223]. They are more commonly naked, but sometimes either generally or partially hairy. Thus in Cicindela, the last joint but one is usually planted with long snow-white bristles in a double series, while the rest of the joints have none; and in Copris Latr. all of them are extremely hairy. In shape they do not vary so much as the maxillary palpi, being most frequently filiform or subclavate, and sometimes setaceous; the last joint varies more in shape than the rest, and is often remarkably large, triangular, and shaped like the head of a hatchet[1224]; and at others it resembles the moon in her first quarter[1225]. In the great dragon-fly, or demoiselle if you prefer the gentler French name (Æshna F.) the labial palpi, which are without any visible joints, are terminated by a minute mucro or point[1226]. With regard to their direction and flexure, they frequently, as in the instance just mentioned, turn towards each other, and lie horizontally upon the end of the labium. Sometimes, as in the Cicindelidæ, they appear to point towards the tail of the insect, the last joint rising, and forming an angle with the rest of the feeler. In other instances they diverge laterally from the labium, the last joint turning again towards it at a very obtuse angle.
4. Mandibulæ[1227].—Having considered the analogues of the lips in our little beings, I must next call your attention to the representatives of the jaws. The vertebrate animals, you know, are mostly furnished with a single pair of jaws, one above and the other below, in which the teeth are planted and which have a vertical motion. But insects are furnished with two pair of jaws, a pair of upper-jaws and a pair of under-jaws, which have no teeth planted in them, and the motion of which is horizontal.—I shall begin with an account of the upper-jaws. These by modern Entomologists, after Fabricius, are denominated mandibles (mandibulæ): a term appropriated by Linné to the beaks of birds. The upper-jaws of insects this great naturalist named maxillæ—and not improperly, since the office of mastication is more peculiarly their office than that of the under-jaws, which Fabricius has distinguished by that name: as the term mandible, however, is generally adopted, I shall not attempt to disturb it.
The mandibles close the mouth on each side under the labrum or upper-lip. They are generally powerful organs, of a hard substance like horn; but in the Lamellicorn beetles of Mr. MacLeay's families of Scarabæidæ and Cetoniadæ, they are soft, membranous, and unapt for mastication. In Coleopterous insects they commonly articulate with the head by means of certain apophyses or processes, of which in many cases there are three discoverable at the exterior base of the mandibles; one, namely, at each angle, and one in the middle. That on the lower side is usually the most prominent, and wears the appearance of the condyle of a bone: it is received by a corresponding deep socket (or cotyloid cavity) of the cheek, in which, being perfectly smooth and lubricous, it moves readily, but without synovia, like a rotula in its acetabulum. The upper one projects from the jaw, forms the segment of a circle, and is concave also on its inner face. A corresponding more shallow, or, as anatomists speak, glenoid cavity of the cheek, where it meets the upper-lip, receives it, and the concave part admits a lubricous ball projecting from the cheek, upon which it turns[1228]. This structure you will find in the stag-beetle, and some other timber-devourers. Other Coleoptera have only a process of a similar structure at each of the dorsal angles of the base of the mandible, the intermediate one being wanting; and the articulation does not materially differ, as far as I have examined them, in the Hymenoptera and Neuroptera. In the Orthoptera, the structure approaches more nearly to that of the stag-beetle, since there are three prominences: it is thus well described by M. Marcel de Serres: "This articulation," says he, "takes place in two ways. At first, in the upper surface of the mandible, and at its base, may be observed two small prominences and a glenoid cavity; these prominences are received in two glenoid cavities excavated in the shell of the front, as the cavity of the mandible receives a small prominence of the same part. Below the mandible, and at its base, there is a kind of condyle, which has its play in a cotyloid cavity excavated in the shell of the temple, far below the eye, and at the extremity of the coriaceous integument of the head[1229]." Within the head in this order, at least in Locusta Leach, is a vertical septum, which divides the head into two chambers, as it were, an occipital and a frontal, consisting of a concave triangular stem, terminating in two narrower concave triangular branches, so as to resemble the letter Y, and forming three openings, an upper triangular one, and two lateral subquadrangular ones, which last are the cavities that receive the base of the mandibles. This partition, which I would name Cephalophragma, doubtless affords a point of attachment to many of the muscles of the head. It does not appear to have been noticed, unless it be synonymous with the intermaxillary arcade of Marcel de Serres[1230]. Probably a corresponding support to the muscles, &c. may exist, as we have seen it does in Vespa L.[1231], in many other heads of the different Orders, which have not yet fallen under examination. Many mandibles, as those of the hornet &c., appear to be suspended to the cavity of the head on the inside by a marginal ligament sufficiently relaxed to admit of their play: those of the Orthoptera, M. Marcel de Serres informs us, are united to the head by means of two cartilages, the outermost being much the shortest, to which their moving muscles are attached. These he considers as prolongations of the substance of the mandible[1232]. The bottom of mandibles, when cleared of the muscles &c., inclines almost universally to a triangular form; but in some cases, as in the stag-beetle, it is nearly a trapezium. I cannot conclude this subject without noticing the motions of the mandibles. What the author lately quoted has said with regard to those of the Orthoptera, will, I believe, apply equally well to all the mandibulate orders. "The articulation of mandibles with the skull appears to take place by two points solely; and as these parts only execute movements limited to a certain direction, they may be referred to ginglymus[1233].—The movements of mandibles are limited to those from within outwards, and from without inwards[1234]." Whether they are restricted from any degree of vertical motion, has not yet been proved, as the jaws of vertebrate animals move horizontally as well as vertically—so those of insects may have some motion vertically as well as horizontally; and it seems necessary for some of their operations that they should. I am not anatomist enough to speak with confidence on the subject, but the ball and socket articulation at the lower part of the mandible, and the curving one at the upper, though a kind of ginglymus, seems to imply a degree of rotatory movement, however slight.
I must next say something upon the general shape of these organs. Almost universally they incline to a triquetrous or three-sided figure, with their external surface convex, sometimes partially so, and their internal concave. Most frequently they are arched, curving inwards; but sometimes, as in Prionus octangularis[1235], a Capricorn beetle, and others of that genus, they are nearly straight; and in Rhina barbirostris[1236], a most remarkable Brazilian weevil, their curvature is outwards. In Pholidotus lepidotus MacLeay, and Lucanus Elephas, two insects of the stag-beetle tribe, they are bent downwards; and in Lucanus nebulosus K. (Ryssonotus MacLeay) they turn upwards[1237]. They are usually widest at the base, and grow gradually more slender to the apex, but in the hornet (Vespa Crabro) the reverse takes place, and they increase in width from the base to the apex; and in the hive-bee, and others of that tribe, they are dilated both at base and apex, being narrowest in the middle; others are nearly of the same width every where. In those insects that use their mandibles principally for purposes connected with their economy, they are often more broad in proportion to their thickness, than they are in those which use them principally for mastication. In the locust tribes (Locusta Leach), they are extremely thick and powerful organs, and fitted for their work of devastation; but in the glow-worm (Lampyris), they are very slender and minute. In those brilliant beetles, the Buprestes, they are very short; but in the stag-beetles, and those giants in the Capricorn tribe, the Prioni, they are often very long[1238]. They either meet at the summit, lap over each other, cross each other, or are protended straight from the head; as you have doubtless observed in the stag-beetle, whose terrific horns are mandibles of this description. These organs are usually symmetrical, but in some instances they are not: thus in Hister lævus, a kind of dung-beetle, the left hand mandible is longer than the right; in Creophilus maxillosus K. (Staphylinus L.), a common rove-beetle, in the left hand mandible the tooth in the middle is bifid, and in the right hand one intire; and in Bolbocerus K. the mandible of one side, in some species the dexter, and in others the sinister, has two teeth, and the other none.
The next circumstance with respect to these organs which demands our attention, is the teeth with which they are armed. These are merely processes of the substance of the mandible, and not planted in it by gomphosis[1239], as anatomists speak, as they are in vertebrate animals. They have, however, in their interior, at the base at least, in the Orthoptera, a coriaceous lamina that separates them in some sort from the body of the mandible[1240]. Many insects, however, have mandibles without teeth; some merely tapering to a sharp point, others obtuse at the end, and others truncated[1241]. Of those that have teeth, some have them on the inside at the base, as Manticora, an African tiger-beetle[1242]; others in the middle, as Staphylinus olens, a rove-beetle, Lethrus cephalotes, &c.[1243]; others at the end, as many weevils (Curculio L.)[1244]; others again on the back, as the Rutelidæ, a tribe of chafers[1245], and Lethrus, a beetle just named; others once more on the lower side of the base, in the form of a tooth or spine, as in Melitta spinigera, a species of wild-bee, and some of its affinities[1246]; and lastly, others on the upper side of the base in the form of a long tortuous horn, as in that singular wasp Synagris cornuta F. before noticed as a sexual character[1247]. In the stag-beetle tribes (Lucanus L.) these teeth are often elongated into short lateral branches, or a terminal fork[1248]. They are sometimes truncated, sometimes obtuse, and sometimes acute.
But with regard to their kind, it will be best to adopt the ideas of M. Marcel de Serres; for though his remarks are confined to the Orthoptera, they may be applied with advantage to the teeth that arm the mandibles of insects in general. He perceives an analogy between those of this Order and the teeth of quadrupeds; and therefore divides them into incisive or cutting, laniary or canine, and molary or grinding teeth. He denominates those incisives that are broad, having in some degree the shape of a wedge, their external surface being convex, and their internal concave; whence they are evidently formed for cutting. The laniaries are those which have a conical shape, are often very acute, and in general the longest of any; and in some insects, as the carnivorous Orthoptera (and the Libellulina), they cross each other. The molaries are the largest of all, and their purpose is evidently to grind the food. There is never only a single one to each mandible, while the number of the incisives and laniaries is very variable. As the molaries act the principal part in mastication, they are nearer the inner base of the mandible or point of support: they serve to grind the food, which has been first divided by the incisives or torn by the laniaries. The carnivorous tribes are destitute of them; in the omnivorous ones they are very small, and in the herbivorous ones they are very large[1249]. So that in some measure you may conjecture the food of the animal from the teeth that arm its mandibles. Of incisive teeth you may find an example in those that arm the end of the mandibles of most grasshoppers (Locusta), and of the leaf-cutter-bees (Megachile Latr.)[1250]; of the laniary or canine teeth, you will find good examples in the mandibles of the dragon-flies (Libellulina); the two external teeth of the apex of those of the leaf-cutter bees may be regarded as between the incisives and laniaries; and the pointed mandibles without teeth may be deemed as terminating in a laniary one[1251]. The lower part of the inner or concave surface of the mandibles of grasshoppers will supply you with instances of the molary teeth, and the apex, also, of those of some weevils, as Curculio Hancocki K.[1252] But the most remarkable example of a molary organ is exhibited by many of the Lamellicorn beetles, especially those that feed upon vegetables, whether flower or leaf.—Knoch, who indeed was the first who proposed calling mandibles according to their teeth, incisive, laniary, or molary, but who does not explain his system clearly, observed that the mandibles of some Melolonthæ have a projection with transverse, deep furrows, resembling a file, for the purpose of bruising the leaves they feed upon[1253]: and M. Cuvier, long after, observed that the larvæ of the stag-beetle have towards their base a flat, striated, molary surface; though he does not appear to have noticed it in any perfect insect[1254]. This structure, with the exception of the Scarabæidæ and Cetoniadæ, seems to extend very generally through the above tribe; since it may be traced even in Geotrupes, the common dung-chafer, in which at the base of one mandible is a concave molary surface, and in the other a convex one, but without any furrows: a circumstance that often distinguishes those that have furrows.—In the Dynastidæ the affinity of structure with the Melolonthidæ &c. is more pronounced, the furrows to which ridges in the other mandible correspond being reduced to one or two wide and deep ones; whereas in some of the latter tribe they are very numerous. These mandibles, in many cases, at their apex are furnished with incisive teeth to cut off their food, and with miniature mill-stones to grind it[1255]. The part here alluded to I call the Mola.
Were I to ask you what your idea is with regard to the use of the organs we are considering, you would perhaps reply without hesitation, "Of what possible use can the jaws of insects be but to masticate their food?" But in this you would in many instances be much mistaken; as you will own directly if you only look at the mandibles of the stag-beetle—these protended and formidable weapons, as well as those of several other beetles, cannot be thus employed. "Of what other use, then, can they be?" you will say. In the particular instance here named, their use, independent of mastication, has not been satisfactorily ascertained; but in many other cases it has. Recollect, for instance, what I told you in a former letter, of those larvæ that use their unguiform mandibles as instruments of motion[1256]. Again: amongst the Hymenopterous tribes, whose industry and varied economy have so often amused and interested you, many have no other tools to aid them in their various labours and mechanical arts: to some they supply the place of trowels, spades, and pick-axes; to others that of saws, scissors, and knives—with many other uses that might be named. In fact, with the insects of this intire Order mastication seems merely a secondary, if it is at any time their use. Still comprehending in one view all the mandibulate Orders, though some use their mandibles especially for purposes connected with their economy, yet their most general and primary use is the division, laceration, and mastication of their food; and this more exclusively than can be affirmed of the under-jaws (maxillæ). This will appear evident to you, when you consider that insects in their larva state, in which universally their primary business is feeding, with very few exceptions use the organs in question for the purpose of mastication, even in tribes, as the Lepidoptera, that have only rudiments of them in their perfect state—while the maxillæ ordinarily are altogether unapt for such use. The exceptions I have just alluded to are chiefly confined to the instance of suctorious mandibles; or those which, being furnished at the end with an orifice, the animal inserting them into its prey, imbibes their juices through it. This is the case with the larvæ of some Dytisci, Hemerobius, and Myrmeleon[1257]; and spiders have a similar opening in the claw of their mandibles, which is supposed to instil venom into their prey[1258].