ix. The parts and appendages of the head are now in the last place to be considered. I shall begin with the Mouth, or rather the orifice in which the trophi or organs of manducation are inserted. In some insects, as was before observed, they occupy all the under-side of the head, as in the Arachnida, Myriapoda, &c; but in the great majority they fill an orifice in its anterior part, which in some instances, as in Lampyris, the Lepidoptera, Cimex L., Truxalis, appears to be nearly under the head; but in general it terminates that part, though it extends further below than above. In Chermes, a Homopterous genus, the promuscis is stated to be in the Antepectus, and consequently the mouth; but, as I shall endeavour to prove to you hereafter, this is a fallacy. In the males of the species of Coccus there is no mouth at all. In that of the elm (C. Ulmi) in lieu there are ten little shining points, arranged two before and two behind in a line, and three on each side in a triangle[1181]. It is to be observed that the orifice of which I am speaking is usually much smaller in those insects which take their food by suction, the Hemiptera, Lepidoptera, Diptera, &c., than in the masticating tribes. With regard to the real mouth, or that through which the food enters, I have nothing at present to observe, except that it lies between the upper-lip and tongue, is sometimes covered by a valve, as in Apis, Vespa[1182], &c., and is different in the masticators and suckers.
I shall next offer a few observations seriatim, as they stand in the Table, upon the organs of manducation; which, to avoid circumlocution, instead of Instrumenta cibaria, the name Fabricius gave them, I shall call trophi or feeders. It is upon these parts, you are aware, that the system of the celebrated Entomologist just mentioned is founded; and could they always, or even for the most part, be inspected with ease, they would no doubt afford characters as various and discriminative as those of the vertebrate animals. Differences in these parts indicate a difference in the mode in which the animal takes its food, and often in the kind of food, and sometimes in its general economy and habits,—circumstances which are powerful and weighty in supporting the claim of any set of animals to be considered as forming a natural genus or group. Trifling variations, however, of these parts, unless supported by other characters and qualities, ought not to have much stress laid upon them, since, if we insist upon these, in some tribes almost every species might be made a genus.
With respect to their trophi in general, insects of late have been divided into two great tribes[1183], masticators and suckers; the first including those that are furnished with instruments to separate and masticate their food; namely, an upper- and under-lip (labrum and labium), upper- and under-jaws (mandibulæ and maxillæ), labial and maxillary palpi, and a tongue (lingua): and the second those in which these parts are replaced by an articulate or exarticulate machine, consisting of several parts and pieces analogous to the above, which pierce the food of the animal, and form a tube by which it sucks its juices. If, however, the mode in which insects take their food be strictly considered, it will be found that in this view they ought rather to be regarded as forming three tribes; for the great majority of the Hymenoptera order, and perhaps some others, though furnished with mandibles and maxillæ, never use them for mastication, but really lap their food with their tongue: these, therefore, might be denominated lappers.
When a mouth is furnished with the seven ordinary organs used in taking the food and preparing it for deglutition—I mean the upper-lip and the two upper-jaws; the under-lip and the two under-jaws, including the labial and maxillary palpi; and the tongue—I denominate it a perfect mouth; but when it is deficient in any of these organs, or they exist merely as rudiments, or when their place is supplied by others, (which, though they may be analogous parts, have little or no connection with them in their use or structure,) I denominate it an imperfect mouth. This last I would further distinguish, according to the nature of its trophi, by other and more distinctive terms, as I shall presently explain to you.
1. Labrum[1184].—I shall first consider the organs present in a perfect mouth, beginning with the upper-lip (labrum). This part, which Fabricius sometimes confounded with the nose, miscalling it clypeus, is usually a moveable[1185] piece, attached by its base to the anterior margin of the part last named, and covering the mouth, and sometimes the mandibles, from above. In insects in their last state it is usually of a horny or shelly substance; yet in some cases, as in Copris and Cetonia, beetles that imbibe juices, it is membranous. In form and shape it varies greatly, being sometimes nearly square, at others almost round; in some insects representing a parallelogram, in others a triangle, and in many it is oblong. In some instances it is long and narrow, but more generally short and wide. It is often large, but occasionally very minute. In the majority it has an intire margin, but it is not seldom emarginate or bilobed, or even dentate. Its surface is commonly even, but it is sometimes uneven, sometimes flat, at others convex, and in a few species armed with a short horn or tubercle[1186]. As to its pubescence, it is often naked, but now and then fringed or clothed with down or hairs, or sprinkled with bristles. It consists in almost every instance of a single piece; but an exception to this occurs in Halictus, a little bee, in the females of which it is furnished with a slender appendage[1187].—The direction of the upper-lip is various. It is rarely horizontal, or in the same line with the nose, often vertical; it sometimes forms an obtuse angle with the anterior part of the head, and occasionally an acute one, when it is more or less inflexed. The use of this part is ordinarily to close the mouth from above, to assist in retaining the food while undergoing the process of mastication; but in many Hymenopterous insects its principal use seems to be, to keep the trophi properly folded; and in some cases where it is inflexed, as in the leaf-cutter bees (Megachile Latr.), to defend its base, while the mandibles are employed, from injury by their action[1188].
2. Labium[1189].—On the under-side of the head, and opposed to the upper-lip, the mouth is closed by another moveable organ, concerning the nomenclature and analogies of which Entomologists have differed considerably. At the first view of it, this organ seems a very complex machine, since the under-jaws or maxillæ are attached to it on each side, and the tongue is often seen to emerge from it above, so as to appear merely a part of it; but as the former answer to the upper-jaws, and the latter is the analogue of the part bearing the same name in the vertebrate animals, I shall consider these as distinct and primary organs, and treat of the under-lip (labium) of which I am now speaking, by itself. Linné takes no notice of this part, but his illustrious compatriot and cotemporary, De Geer, did not so overlook it: he appears to consider the whole apparatus, including the maxillæ, as the labium[1190]; but sometimes he distinguishes the middle piece by that name[1191]; and the tongue, in the case of the stag-beetle, he denominates a proboscis (trompe)[1192]. In the Hymenoptera he calls this part tongue, under-lip, and proboscis: but seems to prefer the last term[1193]. Fabricius originally regarded the whole middle piece as a labium[1194]; but afterwards (though his definition is not accurate, since he assigns the palpi to the ligula, which he affirms is covered by the labium—circumstances by no means universal in Coleoptera) he considers this as consisting of ligula and labium[1195]. Latreille at first regarded the ligula of Fabricius as the labium, and called the labium of that author the mentum[1196]; but afterwards he gave the name of labium to the whole middle piece of the lower apparatus of the mouth—calling the upper piece, with Fabricius, the ligula, and retaining the denomination of mentum for the lower[1197].
If the circumstances of the case are duly considered, I think you will be convinced that the term under-lip, or labium, should be confined to that part which the learned Dane so named. For I would ask, Which is the part on the under side of the head that is the antagonist, if I may so speak, of the upper-lip or labrum? Is it not that organ which, when the mouth is closed, meets that part, and in conjunction with it shuts all in? Now in numerous insects, particularly the Lamellicorn beetles (Scarabæus and Lucanus L.), this is precisely the case. In the Predaceous beetles, indeed, (Cicindela, Carabus, Dytiscus L. &c.) the under-lip does not meet the upper, to close the mouth and shut in the tongue; neither can the tongue be said so to do, but only, from some circumstances connected with its manner of taking its food, it is requisite that the last-mentioned organ should not be retractile or covered; but its miscalled mentum is still the analogue of that part which helps to close the mouth in the former tribe. Should not this, therefore, which so often performs the office, be distinguished by the name, of a lip? Again, is it not rather incongruous to consider that organ which confessedly more or less performs the office of a tongue, as a part of the lip? Though it often wears that appearance, yet I believe, if the matter is thoroughly and patiently investigated, it will be found that on their upper side its roots are attached to the interior of the upper side of the head, as well as on their lower side to the labium; so that it may be regarded as common to the two lips, and therefore not properly considered as an appendage of the under-lip alone.
Having assigned my reasons for preferring the name given to the part in question by Fabricius, rather than that of Latreille, I shall next make my observations on the part itself. In many cases the labium, or the middle piece of the lower oral apparatus, appears to consist of two joints: this you may see in the great water-beetle (Hydrophilus piceus), the burying-beetles (Necrophorus), the Orthopterous tribes[1198], the Hymenoptera[1199], and others. In this case the upper or terminal piece is to be regarded as the labium, and the lower or basal one (which Mr. MacLeay calls the stipes) as the mentum or chin—at other times, as in some Lamellicorn beetles, the only separation is a transverse elevated line, or an obtuse angle formed by the meeting of the two parts, and very frequently there is no separation at all, in which case the whole piece, the mentum merging in it, may be denominated the labium.