Under this head I must not pass without notice an appendage of the mandibles, to be found in some of the rove-beetles (Staphylinidæ), as in Ocypus, Staphylinus, and Creophilus Kirby. In the first of these it is a curved, narrow, white, subdiaphanous, submembranous, or rather cartilaginous piece, proceeding from the upper side of the base of the mandible[1259]; in the second it is broader, straighter, and fringed internally and at the end with hairs; and in this at first it wears the appearance of being attached laterally to the mandible under the tooth[1260], but if closely examined, you will find that it is separate: in Creophilus maxillosus it is broader. This is the part I have named prostheca. It is perhaps useful in preventing the food from working out upwards during mastication.
5. Maxillæ[1261]. The antagonist organs to the mandible in the lower side of the head, are the under-jaws, or maxillæ—so denominated by the illustrious Entomologist of Kiel. Linné appears to have overlooked them, except in the case of his genus Apis, in which he regards them, and properly, as the sheath of the tongue. De Geer looked upon them in general as part of the apparatus of the under-lip or labium; and such in fact they are, as will appear when we consider them more particularly. Fabricius has founded his system for the most part upon these organs, the principal diagnostic of ten out of his thirteen Classes (properly Orders) being taken from them; and in the modern, which may be termed the eclectic, system, although the Orders are not founded upon them, yet the characters of genera, and sometimes of large tribes, are derived from them: and as they appear less liable to variation than almost any other organ, as Mr. W. S. MacLeay has judiciously observed, there seems good reason for employing them—it is therefore of importance that you should be well acquainted with them.
Their situation is usually below each mandible, on each side of the labium; towards which they are often somewhat inclined, so that their tips meet when closed. In some cases, as in the Predaceous beetles (Carabus L. &c.), they exactly correspond with the mandibles; but in others their direction with respect to the head is more longitudinal, as in the Hymenoptera, &c. In substance they may be generally stated to be less hard than those organs; yet in some instances, as in the Libellulina, Anoplognathidæ, &c. they vie with them, and in the Scarabæidæ and Cetoniadæ exceed them, in hardness. In the bees, and many other Hymenoptera, they are soft and leathery. Their articulation is usually by means of the hinge on which they sit: it appears entirely ligamentous, and they are probably attached to the labium at the base, or mentum—at least this is evidently the case with the Hymenoptera, in which the opening of the maxillæ pushes forth the labium and its apparatus. In that remarkable genus related to the glow-worms, now called Phengodes (Lampyris plumosa F.), and in the case-worm flies (Trichoptera K.), the maxillæ appear to be connate with the labium, or at least at their base.—As to their composition, these organs consist of several pieces or portions. At their base they articulate with a piece more or less triangular, which I call the hinge (Cardo)[1262]. This on its inner side is often elongated towards the interior of the base of the labium, to which it is, as I have just observed, probably attached. This elongate process of the hinge in Apis, Bombus, &c. appears a separate articulation; and the two together form an angle upon which the mentum sits[1263], and by this the maxilla acts upon the labial apparatus.
The next piece is the stipes or stalk of the maxilla. This is the part that articulates with the hinge, and may be regarded in some cases, as in the Orthoptera &c., as the whole of the maxilla below the feeler; and in others, as in the Geotrupidæ, Staphylinidæ &c., as only the back of it, the inside forming the lower lobe. This piece is often harder and more corneous than the terminal part, is linear, often longitudinally angular, and in the bee-tribes (Apis L.) is remarkable on its inner side for a series of bristles parallel to each other like the teeth of a comb[1264]. In Pogonophorus Latr., a kind of dor or clock-beetle, it is armed on the back with four jointed spines, the intermediate one being forked[1265]. M. Latreille has thus described the stipes of the maxillæ of Coleoptera: "Next comes the stalk," says he, "which consists of three parts: one occupies the back and bears the feeler; the second forms the middle of the anterior face, and its figure is triangular; the third fills the posterior space comprised between the two preceding; and is that which is of most consequence in the use of the maxilla; the anterior feeler, where there are two, the galea, and the other appendages that are of service in deglutition, are part of that piece[1266]."
The third and terminal portion of the maxilla is formed by the lobe, or lobes (Lobi). This may be called the most important part of the organ, since it is that which often acts upon the food, when preparing for deglutition. When armed with teeth or spines at the end, its substance is as hard as that of the mandibles; but when not so circumstanced, it is usually softer, resembling leather, or even membrane[1267]; and sometimes the middle part is coriaceous, and the margin membranous. This part is either simple, consisting only of one lobe, as you will find to be the case with the Hymenoptera, Dynastidæ, Nemognatha, and several other beetles; or it is compound, consisting of two lobes. In the former case, the lobe is sometimes very long, as in the bee tribes, and the singular genus of beetles mentioned above[1268], Nemognatha; and at others very short, as in Hister, &c. The bilobed maxillæ present several different types of form. Nearest to those with one lobe are those whose lower lobe is attached longitudinally to the inner side of the stalk of the organ, above which it scarcely rises. Of this description is the maxilla in the common dung-beetle (Geotrupes stercorarius), and rove-beetle (Staphylinus olens).[1269] Another kind of formation is where the lower lobe is only a little shorter than the upper: this occurs in a kind of chafer (Macraspis tetradactyla MacLeay).[1270] A third is where the upper lobe covers the lower as a shield; as you will find in the Orthoptera order, and the Libellulina, and almost in Meloe[1271]. A fourth form is where the upper lobe somewhat resembles the galeate maxilla just named; but consists of two joints. This exists in Staphylinidæ, &c.[1272] The last kind I shall notice is when the upper lobe not only consists of two joints, but is cylindrical, and assumes the aspect of a feeler or palpus[1273]. This is the common character of almost all the Predaceous beetles (Entomophagi Latr.). This lobe, which has been regarded as an additional feeler, is strictly analogous to the upper lobe in other insects, and therefore should rather be denominated a palpiform lobe than a palpus. Where there are two lobes, the upper one is most commonly the longest; but in many species of the tribe last mentioned the lower one equals or exceeds it in length[1274].
The lobes vary in form, clothing, and appendages. The upper palpiform lobe in those beetles just mentioned, in general varies scarcely at all in form; but the genus Cychrus (which is remarkable for a retrocession from the general type of form of the Carabi L. making an approach towards that of those Heteromera which, from their black body and revolting aspect, Latreille has named Melosomes,) affords an exception, the upper joint being rather flat, linear-lanceolate, incurved, and covering the lower lobe[1275], which it somewhat resembles. The lower lobe also in this tribe varies as little as the upper, being shaped like the last joint of that lobe in Cychrus just described, except that in Cicindela it is narrowest in the middle[1276]. In other tribes the upper valve is sometimes linear and rounded at the apex, and the lower truncated, as in Staphylinus olens[1277]; sometimes the upper one is truncated or obtuse, and the lower acute, as in Trogosita and Parnus[1278]. In Ptinus, another tribe of beetles, before noticed as injurious to our museums[1279], the reverse of this takes place, the upper-lobe, which is the smallest and shortest, being acute, and the lower truncated[1280]. In Blaps both are acute[1281]. In Rhipiphorus and Scolytus the lobes are nearly obsolete. The lower lobe is bifid in Languria, a North American genus of beetles, so as to give the maxilla the appearance of three lobes[1282]; and in Erotylus, a South American one, the upper is triangular[1283]: it is often oblong, quadrangular, linear, &c. in others.—In those that have only one lobe the shape also varies. In Gyrinus, the beetle that whirls round and round on the surface of every pool, which, though it belongs to the Predaceous tribe, has only one lobe, the lobe represents a mandible in shape of the laniary kind, being trigonal and acute[1284]; and in the Anoplognathidæ, a New Holland tribe of chafers, in which it is, as it were, broken, the lobe forming an angle with the stalk, it is concavo-convex and obtuse, and somewhat figures a molary tooth[1285]. In the first tribe into which the bees (Apis L.) have been divided (Melitta Kirby), the lobe is often linear or strap-shaped, and bifid at the apex; and in the second (Apis K.) lanceolate and intire[1286]. In Cerocoma it is long and narrow[1287]. More variations in form might be named, but these are sufficient to give you a general idea of them in this respect. With regard to their clothing, I have not much to observe—in examining the Predaceous beetles you will observe, that the interior margin of the lower incurved lobe is fringed with stiff bristles or slender spines, and in many other beetles either one or both lobes have a thick coating or brush of stiffish hairs[1288]; but in several cases only the apex of the lobe is hairy. In the Orthoptera order, and many of the Melolonthidæ or chafers, the whole maxilla is without hairs, or nearly so.
The appendages of the maxillæ are next to be noticed. These are principally their claws, or laniary teeth; for they are seldom armed with incisive or molary teeth. The whole tribe of Predaceous beetles, with few exceptions, have the inner lobe of their maxilla armed with a terminal claw, which in the Cicindelidæ articulates with the lobe, and is moveable, but in the rest of the tribe is fixed[1289]. In Phoberus MacLeay the lower lobe has two spines[1290]. In Locusta this lobe has three or four spines or laniary teeth, and in Æshna there are six, which, like the claw of Cicindela, are moveable[1291]. In others both lobes terminate in a single spine or claw: this is the case with Paxillus MacLeay[1292]. In Passalus, nearly related to the last genus, the upper lobe is armed with a single spine, and the lower one with two[1293]. Those maxillæ that terminate in a single lobe are also often distinguished by the spines or teeth with which it is armed; thus in a nondescript chafer belonging to the Dynastidæ (Archon K. MS.) it terminates in two short teeth; in that remarkable Petalocerous genus Hexodon Oliv. in three truncated incisive ones[1294]; in Dynastes Hercules in three acute spines[1295]. Four similar ones arm the apex of the maxilla in that tribe of Rutelidæ which have striated elytra; and five that are stout and triquetrous those of Melolontha Stigma F. Many others have six spines, sometimes arranged in a triple series[1296]. Besides teeth or spines, in some cases the lobes of maxillæ terminate in several long and slender laciniæ or lappets fringed with hairs. At least those of a Leptura (L. quadrifasciata L.) described by De Geer, appear to be thus circumstanced. He conjectures that this beetle uses its maxillæ to collect the honey from the flowers[1297].
As the principal use of the mandibles is cutting and masticating, so that of the organs we are considering seems to be primarily that of holding the food and preventing it from falling while the former are employed upon it. I say this is their primary use; for I would by no means deny that they assist occasionally in comminuting or lacerating it. In fact, were there no organs appropriated to this use, and if both mandibles and maxillæ were employed at the same time in comminuting the food, it seems to me that it must fall from the mouth. In a large proportion of insects the lobes of the maxillæ are not at all calculated for laceration or comminution; and in those tribes—as the Melolonthidæ, Rutelidæ, Dynastidæ—in which they seem most fitted for that purpose, the mandibles have incisive teeth at their apex, and at their base a powerful mola or grinder: circumstances which prove, that even in this case the business of mastication principally devolves upon them.