It may here be observed, that in the promuscis the elongation of the organs seems to be made chiefly at the expense of all the palpi, but in the proboscis at that of the labial only; and in some cases at that also of the mandibles or maxillæ;—the former merging in the labrum and the latter in the labium.
iii. Antlia[1387].—The third kind of imperfect mouth is that of the Lepidoptera, which I have called Antlia. Fabricius denominates it lingua: but as this organ has no analogy with the real tongue of insects, this is confessedly improper, and it appeared necessary therefore to exchange it for another denomination: I have endeavoured to apply a term to it that indicates its use—to pump up, namely, the nectar of the flowers into the mouth of the insect. On a former occasion I described to you the structure of this instrument[1388]; but further discoveries with regard to it having since been made by MM. Savigny and Latreille, I shall here give you the result of their observations. The former of these able physiologists has detected in the mouth of the Lepidoptera rudiments of almost all the parts of a perfect mouth. Of the correctness of this assertion you may satisfy yourself, if you consult his admirable elucidatory plates, and compare them with the insects. Just above the origin of the spiral tongue or pump, the head is a little prominent and rounded; and immediately below the middle of this prominence there is a very minute, membranous, triangular or semicircular piece; which from its position, as covering the base of the antlia, may be regarded as the rudiment of the upper-lip (labrum)[1389]. On each side of the outer base of the antlia is another small immoveable piece, resembling a flattened tubercle, the end of which is internally hairy or scaly: these pieces appear to represent the mandibles[1390]. Near the base of each half of the antlia, just below a sinus, may be distinctly seen the minute, usually biarticulate rudiment of a maxillary palpus[1391]; demonstrating to a certainty that these spiral organs, at least their lateral tubes or Solenaria, are real maxillæ[1392]. The rudiment of the under-lip (Labium) is the almost horny triangular piece united by membrane to the two stalks of the maxillæ, and supporting at its base the recurved labial palpi; which are so well known that I need not enlarge upon them[1393]. Amongst these parts there seems at first sight no representative of the tongue; but M. Latreille has advanced some very ingenious, and I think satisfactory arguments[1394], which go to prove that this part, at least the tongue of Hymenoptera, has its analogue in the intermediate tube or Fistula formed by the union of the two maxillæ, and which conveys the fluid aliment of this Order to the pharynx. As in Diptera the maxillæ sometimes merge in the labium, so here the tongue (as it were divided longitudinally) merges in the maxillæ. He further observes, that in a transverse section of the maxilla of the death's-head hawk-moth (Sphinx Atropos), the lateral tube appeared to be divided into two by a membranous partition, and to contain in the upper cavity a small cylindrical tube, which seemed to be a trachea[1395]. To animals that are without lungs, and breathe by tracheæ, suction must be performed in a very different way from what it is by those that breathe by the mouth: and as in the very extended organs in question the fluid has a long space to pass before it reaches the pharynx, in some way or other these lateral tubes may have the power of producing a vacuum in the middle tube, and so facilitate its passage thither. We see, in the antlia, that the maxillæ receive their vast elongation at the expense of all the other organs, except the labial palpi.
iv. Rostrulum[1396].—An animal very annoying to us affords the type of the next kind of imperfect mouth—I mean the flea. Its oral apparatus, which I would name rostrulum, appears to consist of seven pieces. First are a pair of triangular organs, the laminæ, which together somewhat resemble the beak of a bird, and are affixed, one on each side of the mouth, under the antennæ: these represent the mandibles of a perfect mouth[1397]. Next, a pair of long sharp lancets (Scalpella), which emerge from the head below the laminæ: these are analogous to maxillæ[1398]: a pair of palpi, consisting of four joints, are attached to these near their base[1399], which of course are maxillary palpi. And lastly, in the midst of all is a slender setiform organ (ligula), which is the counterpart of the tongue[1400]. Rösel, and after him Latreille, seem to have overlooked this last piece, since they reckon only six pieces in the flea's mouth[1401]: but the hand and eye of our friend Curtis have detected a seventh, as you see in his figure. From this account it appears, that the elongation of the organs of the Aphaniptera Order is at the expense of the labium and its palpi.
v. Rostellum.—So little is known of the composition of the next kind of imperfect mouth, that I need not enlarge upon it. It is peculiar to the louse tribe (Pediculidæ), and it consists of the tubulet (Tubulus), and siphuncle (Siphunculus). The former is slenderer in the middle than at the base and apex, the latter being turgid, rather spherical, and armed with claws which probably lay hold of the skin while the animal is engaged in suction. When not used, the whole machine is withdrawn within the head; the siphuncle, which is the suctorious part, being first retracted within the tubulet, in the same way as a snail retracts its tentacula[1402]. This apparatus seems formed at the expense of all the other organs.
There are some other kinds of imperfect mouth, which, though they seem not to merit each a distinct denomination, should not be passed altogether without notice. The first I shall mention is that of the family of Pupipara Latr. (Hippobosca L.). It consists of a pair of hairy coriaceous valves, which include a very slender rigid tube or siphuncle, the instrument of suction, which Latreille describes as formed by the union of two setiform pieces[1403]. In Melophagus, the sheep-louse, the union of the valves of the sheath is so short, that they appear like a tube; but if cut off they will separate, and show the siphuncle, as fine as a hair, between them. This organ is of a type so dissimilar, as was before observed, to that of the Diptera in general, and approaches so near to that of the dog-tick (Ixodes), that they may be deemed rather apterous insects with two wings, than to belong to that Order; and the circumstance that some of the family are apterous confirms this idea. In fact they are a transition family that connects the two Orders, but are nearest to the Aptera. In Nycteribia the oral organs differ from those of the other Pupipara in having palpi. This also is the case with those of the genus Ixodes, the palpi of which are placed upon the same base with the instrument of suction, than which they are longer: they appear to consist of two joints, the last very long and flat. The instrument of suction itself is formed by three hard rigid laminæ; two shorter parallel ones above, that cover the third, which is longer and broader, and armed on each side with several teeth like a saw, having their points towards the base[1404]. Many of the other Acari L. have mandibles, and several have not: but their oral organs have not yet been sufficiently examined; and from the extreme minuteness of most of them, this is no easy task; nor to ascertain in what points they differ or agree.
If you consider the general plan of the organs of manducation in the vertebrate animals, how few are the variations that it admits! An upper and a lower jaw planted with teeth, or a beak consisting of an upper or a lower mandible with a central tongue, form its principal features. But in the little world of insects, how wonderful and infinite is the diversity which, as you see, in this respect they exhibit! Consider the number of the organs, the varying forms of each in the different tribes, adjusted for nice variations in their uses:—how gradual, too, the transition from one to another! how one set of instruments is adapted to prepare the food for deglutition by mastication; another merely to lacerate it, so that its juices can be expressed; a third to lap a fluid aliment; a fourth to imbibe it by suction—and you will see and acknowledge in all the hand of an almighty and all-bountiful Creator, and glorify his wisdom, power, and goodness, so conspicuously manifested in the structure of the meanest of his creatures. You will see also, that all things are created after a pre-conceived plan; in which there is a regular and measured transition from one form to another, not only with respect to beings themselves, but also to their organs—no new organ being produced without a gradual approach to it; so that scarcely any change takes place that is violent and unexpected, and for which the way is not prepared by intermediate gradations. And when you further consider, that every being, with its every organ, is exactly fitted for its functions; and that every being has an office assigned, upon the due execution of which the welfare, in certain respects, of this whole system depends, you will clearly perceive that this whole plan, intire in all its parts, must have been coeval with the Creation; and that all the species,—subject to those variations only that climate and different food produce,—have remained essentially the same, or they would not have answered the end for which they were made, from that time to this.
Having given you this particular account of the trophi or organs of the mouth of insects, I must now make some observations upon the other parts of the head. I have divided it, as you see in the Table, into face and subface; the former including its upper and the latter its lower surface. Strictly speaking, some parts of the face, as the temples and cheeks, are common to both surfaces; but I do not therefore reckon them as belonging to the subface, which, exclusive of the mouth and its organs, consists only of the throat, and where there is a neck, the gula.
i. Nasus[1405].—I shall consider the parts of the face in the order in which they stand in the Table, beginning with the nasus or nose. Fabricius has denominated this part the clypeus, in which he has been followed by most modern Entomologists. You may therefore think, perhaps, that I have here unnecessarily altered a term so generally adopted, and expect that I assign some sufficient reasons for such a change. I have before hinted that there is good ground for thinking that the sense of smell in insects resides somewhere in the vicinity of this part; and when I come to treat of their senses, I shall produce at large those arguments that have induced me to adopt this opinion: and if I can make out this satisfactorily, you will readily allow the propriety of the denomination. I shall here only state those secondary reasons for the term, which, in my idea, prove that it is much more to the purpose than clypeus. This last word was originally applied by Linné in a metaphorical sense to the ample covering of the head of the Scarabæidæ, and the thoracic shield of Silpha, Cassida, Lampyris, and Blatta: in all which cases there was a propriety in the figurative use of it, because of the resemblance of the parts so illustrated to a shield. But when Fabricius (though he sometimes employs the term, as Linné did, merely for illustration,) admitted it into his orismological table, as a term to represent universally the anterior part of the face of insects to which the labrum is attached (though in some cases he designates the labrum itself by this name), it became extremely inappropriate; since in every case, except that of the Scarabæidæ, the part has no pretension to be called a shield;—so that the term is rather calculated to mislead than illustrate. This impropriety seems at length to have struck M. Latreille, since in a late essay[1406] he has changed the name of this part to Epistomis, a term signifying the part above the mouth. But there are reasons, exclusive of those hereafter to be produced concerning the sense of smell, which seem to me to prove that nasus is a preferable term; not to mention its claim of priority, as having been used to signify this part a century ago[1407]. When we come to consider the terms for the other parts of the head, as lips, jaws, tongue, eyes, temples, cheeks, forehead, &c. the concinnity, if I may so speak, and harmony of our technical language, seem to require that the part analogous in point of situation to the nose of vertebrate animals should bear the same name. And any person who had never examined an insect before, if asked to point out the nose of the animal, would immediately cast his eye upon this part: so that one of the principal uses of imposing names upon parts—that they might be more readily known—would be attained. If it is objected, that calling a part a nose that has not the sense of smell, supposing it to be so, might lead to mistakes—I would answer, that this objection is not regarded as valid in other cases: for instance, the maxillæ are not generally used as jaws, and yet no one objects to the term; because, from their situation, they evidently have an analogy to the organs whose name they bear. But enough on this subject—we will now consider the part itself.