In all cases, the various parts of the mouth may be reduced to six, the names of which deserve remembering, and this will be rendered easier by the accompanying diagram, in which they are all separated and lettered, so as to distinguish each part; these six parts consist of four side-pieces, and two other organs, one above, the other below. The name of the upper organ is the upper lip, or in Latin, labrum; that of the opposite lower one, is the lower lip, or labium. The upper pair, of side-pieces, are called the mandibles, or upper jaws; the pair below are the lower jaws, or maxillæ.
Let us now briefly mention the use of these various parts of the insect's mouth; beginning with the upper lip, (a) or labrum. Unlike that organ in ourselves, the insect's upper lip is generally, in such insects as bite their food, a plate of horny substance, which serves the purpose of an upper covering to the rest of the mouth. Like our own upper lips, however, that is to say, like those of men, at least, it is fringed with a sort of moustache in most cases.
The mandibles, (b) or upper jaws, in the biting insects, are instruments for cutting the food in pieces. When we bite our food, the movement of our jaw is upwards and downwards; but in the insect it is like that of a pair of scissors, or from side to side! These jaws are sometimes frightful-looking instruments, resembling, in the stag-beetle, horns; and their power is proportionally great. Although their general use is to masticate or crush the food for the insect, this apparatus is often employed as a formidable weapon of defence, or offence; and very frequently in the excavation of a nest or retreat for the insect. They are sometimes so constructed, as to look as if they were furnished with several teeth.
The Stag-Beetle, representing its enlarged Mandibles, like horns.
The maxillæ, or lower jaws, are less firm and strong than the preceding; but they are more complicated in their structure, and have attached to their sides remarkably delicate little processes like the antennæ, which are called the palpi, or, less properly, feelers. The lower jaws are much varied in structure in different insects; as may well be imagined, when it is mentioned that the long and beautiful spiral tube of the butterfly, by which it drains the juices of the flower, is, in reality, composed of the maxillæ, or lower jaws. In ordinary cases, the principal use of the lower jaws appears to be to hold the food, while the upper jaws bruise and crush it; but their use, of course, varies with their structure and modifications.
The last part of the mouth is the labium, or lower lip. This also is a very complicated organ, generally serving to close the mouth from beneath, like the upper from above. It boasts, like the same organ in higher animals, of a chin below it, called the mentum, and on each side of it are a pair of feelers, or palpi, like those of the maxillæ. In the centre a little tongue-like projection is visible in the figure, which is sometimes called the tongue, but perhaps not correctly, if by that term is meant the apparatus for tasting. The lower lip is as much varied as the maxillæ.
It would be impossible, without going too far into minute technical details, to explain to the reader how all these various parts of the mouth are so modified, and altered, as to assume the very different appearances presented to us in the insects already mentioned. This pleasure must be reserved for the time when, well acquainted with the generalities of insect history, which alone form the scope and subject of this little work, he feels anxious to study entomology as a science; and he will find a number of excellent books which will then well repay his attentive perusal, and satisfactorily answer all his inquiries on this and other difficult topics connected with this science.
It cannot fail to be noticed, that in this description of the insect's mouth no mention has been made of its organs of taste. Its eye, and probably ears, and means of touch, have all come before us; but where, it will be asked, is its tongue? The little organ mentioned as a part of the insect's lower lip, although strikingly resembling a tongue in some instances, is in others very different from it, and appears in such cases to be quite unfitted to act as the organ of taste. We must, therefore, suppose, that other parts of the lower lip or mouth have the same faculty in addition to the so-called tongue. It is very certain that insects have the faculty of tasting, and also of discriminating between their food; as few will be disposed to question, who will place a plate of salt, and a plate filled with moist sugar, in a sunny window on a fine day; it will soon become evident which will be favoured with the attention of the flies.
Neither has any mention been made of a nose.[W] A common flesh-fly, however, will soon satisfy us, that it is not devoid of some organ or other which answers this purpose. If we cover a piece of meat under a cloth, or put it into a wire safe where it is certain that the fly cannot see it, it will scent it out as readily, or even more so, than a cat or a dog would. Sometimes this insect's nose proves a treacherous guide, for a plant has been described, called the carrion-flower, which diffuses an odour like that of putrid meat, which attracts the fly, and induces it to deposit its eggs thereon, under the idea that its future young will thus be hatched upon a certain source of food; but in vain, for they perish almost as soon as born, finding nothing in the plant whereon to exist.[X] But as to the precise seat of the organ of this sense, opinions are greatly at variance. Some believe it to exist in the spiracles, some in the mouth, and some in a special organ which they call the nose, but which does not seem in reality to fulfil that office, at any rate, in the generality of cases. M. Huber showed the existence of this faculty to lie somewhere about the mouth, by some interesting experiments upon bees. He presented successively to all parts of a bee's body a camel-hair pencil dipped in oil of turpentine, to which most insects are extremely averse, but the bee took no notice of it. He then took a very fine hair pencil, while the bee had extended its proboscis, and presented it to its eyes and antennæ without effect; but when he pointed it near the cavity of the mouth, above the insertion of the proboscis, the creature started back in an instant, quitted its food, clapped its wings, and walked about in great agitation, and would have taken flight if the pencil had not been removed. On this it began to eat again; but on the experiment being repeated, it showed signs of similar discomposure; oil of marjoram produced the same effect, and more promptly and entirely. He then seized several bees, forced them to unfold their proboscises, and stopped their mouths with paste. When this was sufficiently dry to prevent their getting rid of it, he restored them to their liberty; they appeared by no means incommoded at being thus gagged, but moved about and breathed as readily as their companions. Huber then tempted them with honey, and presented to them, near the mouth, oil of turpentine and other odours that they usually have an aversion to; but all produced no sensible effect upon them, and they even walked upon pencils saturated with them!