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.