Chapter XVII: Letter XXXIV: External Anatomy of Insects (2)

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].

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.

6. _Palpi Maxillares_[1298]. There is one circumstance that particularly distinguishes the _maxillæ_ from the mandibles--they are _palpigerous_, as well as the under-lip. The feelers, or palpi, emerge usually from a sinus observable on the back of the _maxillæ_ where the upper lobe and stalk meet. Their articulation does not materially differ from that of the labial palpi. Each _maxilla_ has properly only _one_ feeler; but, as was lately observed[1299], in certain tribes the upper lobe is jointed and palpiform, which has occasioned it to be considered as a feeler, and these tribes have been regarded as having six feelers. The most general rule with regard to the _length_ of the _palpi_ is, that the maxillary shall be longer than the labial; but the reverse often takes place. In many _bees_ the maxillary consist only of a _single_ joint, and are very _short_; while the labial consist of _four_, and are very _long_[1300]: and in some insects (as in _Pogonophorus_ Latr.) the four palpi are of equal length[1301]. The antennæ are most commonly longer than the palpi; but in several aquatic beetles, as _Elophorus_, _Hydrophilus_, &c., whose antennæ in the water are not in use, the organs we are considering are the longest.--As to the _number_ of their articulations, it varies from one to six; which number they are not known to exceed. In each of the Orders a kind of law seems to have been observed as to the number of joints both in the maxillary and labial palpi, but which admits of several exceptions. Thus in the _Coleoptera_, the _natural_ number may be set at _four_ joints for the _maxillary_, and _three_ for the _labial_ palpi: yet sometimes, as in _Stenus_, _Notoxus_, &c., the _former_ have only _three_ joints, and the _latter_, as in _Stenus_ and _Tillus_, only _two_. In the _Orthoptera_ the law enjoins _five_ for the _maxillary_, and _three_ for the _labial_; and to this I have hitherto observed no exception. In the _Hymenoptera_, the rule is _six_ and _four_, but with considerable exceptions, especially as to the _maxillary_ palpi, which vary from _six_ joints to a _single_ one: thus in the hive-bee and the humble-bee, the labials, including the two flat joints or elevators, have four joints, while the maxillaries are not jointed at all[1302]. In _Chrysis_, in which the latter consist of _five_, the former are reduced to _three_. The _Libellulina_ may almost be regarded as having no maxillary palpi, since they exhibit no organ that is distinctly palpiform. It seems to me that the upper lobe of their maxilla, which articulates with the stalk in the same manner as a feeler, may be regarded as an instance in which that lobe and the feeler coalesce into one; and the mucro that proceeds from the lobe has the aspect of an emerging feeler, and corresponds somewhat with the labial one above noticed[1303]. In the remainder of the _Neuroptera_ and the _Trichoptera_, the prevailing number is _five_ and _three_. In the latter there are exceptions, which will furnish good characters for genera. In the _Lepidoptera_ we find _two_, and sometimes _three_, the maxillary being very minute[1304]. The _Diptera_ Order presents two tribes in this respect quite distinct from each other. The most natural number of joints in the maxillary palpi of the _Tipulidæ_, _Culicidæ_, &c. is _four_ or _five_: the last joint, however, in _Tipula_, _Ctenocera_, &c. like that of the antennæ in _Tabanus_ L., appears to consist of a number of very minute joints[1305]; but in the _Asilidæ_ and _Muscidæ_, &c., the number _two_ seems to be most prevalent[1306]. The _labial_ palpi in this order are obsolete.--As to _shape_, the maxillary palpi, as well as the labial, are usually filiform; but in the weevil tribes (_Curculio_ L.) they are most commonly very short and conical[1307]; in the chafers (_Scarabæus_ L.) they usually are thickest at the apex[1308]; in _Megachile_ and _Euglossa_, wild bees, they are setaceous, growing gradually more slender from the base to the summit[1309]: a tribe of small water-beetles (_Haliplus_), the saw-flies (_Tenthredo_ L.), and several other _Hymenoptera_, have them thickest in the middle[1310]. Their most important part, however, and that which varies most in form, is the _terminal_ joint:--of this I have already related some singular instances[1311], and shall now describe a few more. This joint is sometimes acute, at others blunt, at others truncated: in figure it is ovate, oblong, obtriangular, hatchet-shaped, lunate, transverse, conical, mammillate, subulate, branched, chelate, laciniate, lamellate, &c. &c.[1312]: terms which I shall more fully explain to you hereafter, and which I only mention here to show the numerous variations as to figure, of which this joint exhibits examples. The palpi in general at their vertex are often rather concave; and this concavity is formed by a thin papillose membrane, which it is supposed the animal has the power of pushing out a little, so as to apply it to surfaces. The _primary_ use of the palpi of insects will be considered when I treat of their senses; but they probably answer more purposes than one. For instance, when I was once examining, under a lens, the proceedings of a species of _Mordella_, which was busily employed in the blossom of some umbelliferous plant, it appeared to me to open the anthers with its maxillary palpi, and they often held the anther between them: when not so employed, they were kept in intense vibration, more than even its antennæ; and at the same time, as far as I could judge, an _Elater_ made the same use of them.

7. _Lingua_[1313].--This name was applied by Linné to the part in insects representing the _tongue_ in vertebrate animals; and as it performs most of the common offices of a tongue, and the _pharynx_ is situated with respect to it, as we shall presently see, nearly as it is in those animals, there seems no more reason for giving it a new name, than there is for giving a new name to the head or legs of insects, because in some respects they differ from those of the higher animals. I shall not therefore call it _Ligula_, with Fabricius and Latreille, nor _Labium_, with Cuvier and others, but adhere to the original term, which every one understands.

The _tongue_ lies between the two _lips_--the _labrum_ and _labium_. On its upper side, at the base, it meets the palate or roof of the mouth, below which it is attached, it may be presumed, by its roots to the crust of the head, on each side the _pharynx_ or swallow; and on its lower side, in many cases, it is attached to the _labium_, and that very closely, so as to appear to be merely a part of it, and to form its extremity: but in the _Orthoptera_ and _Libellulina_, it is more free, and in form somewhat resembling the tongue of the quadrupeds[1314].--In _substance_ the tongue varies. In general it seems something between membrane and cartilage; but in the Predaceous beetles, in which it is not covered by the _labium_, it approaches nearer to the substance of the general integument, and in _Anthia_ F. it is quite hard and horny:--that just mentioned of the _Orthoptera_ and _Libellulina_ is more fleshy[1315]. With regard to its _station_, in many cases, as in the instance just named, in the Lamellicorn tribe (_Scarabæus_ L.) and others, it is, when unemployed, concealed within the mouth; the lips, mandibles, and maxillæ all closing over it. The tongue of some _Hymenoptera_ also is retractile within the mouth. "When _ants_ are disposed to drink," says M. P. Huber, "there comes out from between their lower jaws, which are much shorter than the upper, a minute, conical, fleshy, yellowish process, which performs the office of a tongue, being pushed out and drawn in alternately: it appears to proceed from the lower-lip.--This lip has the power of moving itself forwards in conjunction with the lower jaws: and when the insect wishes to lap, all this apparatus moves forward; so that the tongue, which is very short, does not require to lengthen itself much to reach the liquid[1316]." M. Lamarck thinks that the _labium_ of insects has a _vertical_ motion (_de haut en bas ou de bas en haut_)[1317]. This it certainly has in some degree; but it has also, as in the above case, a more powerful _horizontal_ one, which is produced, in _Hymenoptera_ at least, by the opening of the maxillæ--as I have already observed[1318].

I have little to say with respect to the _structure_ of the tongue: it generally seems to be without articulations; but in many bees it articulates with the _labium_ where it enters it, so as when unemployed to form a fold with it. In the hive-bee it terminates in a kind of knob or button, which has been falsely supposed to be perforated for imbibing the honey by suction. The upper part of this tongue is cartilaginous, and remarkable for a number of transverse rings: below the middle, it consists of a membrane, longitudinally folded in inaction, but capable of being inflated to a considerable size: this membranous bag receives the honey which the tongue, as it were, laps from the flowers, and conveys it to the _pharynx_[1319]. In _Stenus_ this organ is retractile, and consists of two joints[1320].

The _shape_ of the tongue of insects probably varies as much as any other part; but as it is apt to shrink when dried[1321], and is not easy to come at, we know but little of its various configurations:--in the bees it is very long, in most other insects very short. Though frequently simple and undivided, in many cases it presents a different conformation. Thus in the saw-flies (_Tenthredo_ L.) it terminates in three equal lobes[1322]; in _Stomis_ and _Geotrupes_ in three unequal ones, the intermediate being very short[1323]; in _Carabus_, in three short teeth[1324]; in _Pogonophorus_ it represents a trident[1325]; in the wasp it is bifid, each lobe being tipped with a callosity[1326]; in _Melolontha Stigma_ it is bipartite[1327]; in _Elaphrus_, the analogue of the tiger-beetles, it terminates in a single tooth or point; in the aquatic beetles, _Dytiscus_ L., it is quadrangular and without teeth[1328]; in some _Ichneumonidæ_ it is concavo-convex, and forms a demitube; and in others it is nearly cylindrical[1329].

In many insects it has no _hairs_, but in the Predaceous beetles it generally terminates in a couple of bristles[1330]. In the hive- humble- and other bees, it is extremely hairy[1331]; a circumstance which probably enables it more effectually to despoil the flowers of their nectar. In _Geotrupes stercorarius_, the common dungchafer, and _Melolontha Stigma_ lately mentioned, the lobes of the tongue are fringed with incurved hairs[1332]; and in _Æshna_ it is hairy on the upper side, each hair or bristle crowning a minute tubercle. In many cases the tongue is attended, and sometimes sheathed at the base, by two usually membranous appendages:--these the learned Illiger has denominated _paraglossæ_; and I shall adopt his term. You will find them frequently attached to the tongue of the Predaceous beetles[1333], and to that of many _Hymenoptera_. In the hive-bee and humble-bee they are short, and take their origin within the _labial_ feelers[1334]: in _Euglossa_, another bee, they are long, involute at the tips, and, what is not usual with them, very hairy[1335]: in the wasp, like the lobes of the tongue, they are tipped with a callosity.

Under this head I may observe to you, that the insects whose oral organs we are considering besides a _tongue_ appear likewise to be furnished with a _palate_ (_Palatum_). This, though a part of the roof of the mouth, is not precisely in the situation of the palate of vertebrate animals, since it seems rather the internal lining of the _labrum_. If you take the common dragon-fly (_Æshna viatica_), you will find that the under side of this part and of the _rhinarium_ is lined with a quadrangular fleshy cushion, beset, like the upper surface of the tongue, with minute black tubercles, crowned with a bristle. This cushion is divided transversely into two parts by a depression; the anterior or outer piece being attached to the _labrum_, and the other piece to the _rhinarium_. The former has a central longitudinal cavity, black at the bottom, on the sides of which the tubercles are flat and without a bristle. From its base on each side a spiniform process emerges, forming a right angle with it. These processes seem the antagonists of those mentioned above[1336], that emerge from the _labium_. The posterior or inner piece has on each side a roundish space, attached to the under surface of the two sides of the _rhinarium_, beset also with bristle-bearing tubercles. You will find something similar lining the _labrum_ and _nasus_ of some _Coleoptera_,--say _Geotrupes_, _Necrophorus_, and _Dytiscus_. The first piece I regard as the analogue of the palate, and the second as connected with the sense of _smelling_. In _Necrophorus_ the circular pieces are covered with a finely striated membrane, and in _Dytiscus_ each has a little nipple.

8. _Pharynx_[1337].--On the upper side of the tongue, usually at its base or root, is the _pharynx_, or aperture by which the food passes from the mouth to the _œsophagus_. This orifice, which is situated with respect to the tongue of the _Orthoptera_ and _Libellulina_ nearly as in those insects (at least as far as I have been able to examine them), whose tongue is called a _ligula_ or _labium_,--of course exists in all the mandibulate Orders whose mouth we are now considering. In the _Hymenoptera_ it is covered by a valve, the _Epipharynx_ of Savigny; and it appeared to me to be so likewise in one of the _Harpalidæ_ that I examined. The formation seems different in _Geotrupes_, as far as I can get an idea of it; but it is so difficult to examine the interior of the mouth without laceration of some of the parts, that I can only tell you what the appearances were in one instance, upon removing the _labrum_ from the _mandibles_; and in another, separating the whole apparatus of the _labium_, including the _maxillæ_, from the _mandibles_ and _labrum_. In the former case, the mandibles coincided at the base, the two molary plates (_molæ_), which in this genus are narrow, transverse and not furrowed, are so applied as evidently to have an action upon each other, as the mandible opens and shuts, proper for trituration. Within these is the base of the tongue, under the form of a ventricose sack. The upper part of this last organ, which forms the internal covering of the labium, appears to consist of three (in the recent insect _fleshy_) lobes, the middle one being bent downwards internally, so as to form a kind of sloping cover to an orifice in the part I call the base. After two or three days, the tongue shrinks and dries to a hard substance;--between the mandibles and the base of the tongue I could not discover the _pharynx_. The above apparent opening covered by the tongue was the only one I could perceive. In the latter case, the form and structure of the base of the tongue is more visible: it is an oblong ventricose tubular sack, projecting above anteriorly into an acute angle formed by a fine white membrane, most beautifully and delicately striated with oblique striæ, to be seen only under a powerful lens: on the anterior side of this sack are two parallel cartilaginous ridges close to each other, fringed with short hairs, which take their origin from the angle. I could not be certain whether the orifice covered by the intermediate lobe was only apparent, or real; but I did not succeed in my endeavour to find any other _pharynx_, though from the molary structure of the base of the mandibles one may conjecture that there must be one situated at the base of this sack to receive the food they render after trituration. The excrement of this animal is not fluid. In the _Libellulina_ the _pharynx_ seems closed by two valves meeting. This part in _Hymenoptera_, and probably in other Orders, has the aspect of being cartilaginous and fitted to sustain the action of the substances that have to pass through it[1338].

The _Epipharynx_ is a valve, called by M. Latreille _sublabrum_ (_sous labre_[1339]), attached by its base to the upper margin of the _pharynx_, or that next the _labrum_. In the bees it is said by Reaumur to be of a fleshy substance, and capable of changing its figure. He seems to think it the real _tongue_ of the bee[1340]; but as it does not appear to have any of the uses of a tongue, and merely closes the orifice of the mouth, it surely does not merit that name. M. Savigny calls it a membranous appendage which exactly closes the _pharynx_[1341]. De Geer has examined the _epipharynx_ of the wasp, which he describes as of a scaly substance, and regards merely as the cover of the part just named[1342].

With regard to the _Hypopharynx_, which Latreille considers as a support and appendage of the _epipharynx_, I have little to add to the definition I have given of it above. In the _Libellulina_ the base of the tongue terminates towards the _pharynx_ in a fleshy cushion, armed at each angle next to that part with a short hard horn or tooth of a black colour. This cushion, I suppose, may be analogous to the _hypopharynx_ of M. Savigny[1343]. On the opposite side the pharynx is closed by another fleshy cushion (_epipharynx?_), which appears to line the nose, behind those two mammillæ before described[1344], which form the internal covering of the _rhinarium_.

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.