_Upper-lip_ (LABRUM). The mouth of almost all larvæ, except some of the order _Diptera_, are provided with a distinct _upper-lip_, for retaining their food during mastication. As the construction of this part does not widely differ from that of the perfect insect, which will hereafter be more fully described, it is only necessary to observe, that it is usually a transverse moveable plate, attached posteriorly to the nasus (_clypeus_ F.), and situated just above the mandibles[246].
_Upper-jaws_ (MANDIBULÆ). The most usual figure of these, which are of a hard horny consistence[247], is that of two slightly concave, oblong, or triangular plates, often at their lower extremity of considerable thickness, and of very irregular form, the base of which is filled with powerful muscles, and planted in the side of the mouth so as to move transversely. The other extremity can be made to meet or diverge like the claws of pincers, and are divided into one or more tooth-like indentations, by means of which the food of the larva is cut[248]. This is their construction in the larvæ of all _Lepidoptera_, and in many of those of the other orders. They frequently, however, assume a different form, though their situation is always the same. Thus in the larvæ of the capricorn beetles (_Cerambyx_ L.) and of other wood-boring species, they are shaped like the half of a cone, the inner sides of which, applying close to each other, form a pair of powerful grindstones, capable of comminuting the hardest timber[249]. M. Cuvier has observed, with regard to the mandibulæ of those of stag-beetles (_Lucanus_), that besides their teeth at the extremity, they have towards their base a flat striated molary surface; so that they both cut and grind their ligneous food[250]. It seems to have escaped him, that a similar structure takes place in many perfect insects of the lamellicorn tribe, as I shall hereafter show you. In the larvæ of the water-beetles (_Dytiscus_ L.), ant-lions (_Myrmeleon_ L.), and lace-winged flies (_Hemerobius_ L.), they resemble somewhat the forceps at the tail of an ear-wig, being long and incurved; and, what is more remarkable, hollow and perforated at the end, so as to serve as a channel for conveying into the larva's mouth the juices of the prey which by their aid it has seized. Reaumur even asserts, that the larva of _Myrmeleon_ has no other entrance into its throat than through these tubular mandibles[251]. That of the rove-beetles (_Staphylinus_ L.), and of many other Coleopterous genera, have these organs of this forcipate construction, without being perforated[252]. In the larva of the carnivorous flies, and many other _Diptera_, are two black incurved subulate parts, connected at the base, and capable of being protruded out of, and retracted into, the head, through the skin of which they are usually visible. As I informed you in a former letter[253], these mandibles are used for _walking_ as well as _feeding_: they are parallel to each other, and are neither formed for cutting nor grinding like the mandibles of other insects, but merely detach particles of food by digging into it and tearing the fibres asunder. In this operation they are probably assisted by an acutely triangular dart-like instrument of a horny substance, which in some species (_Musca vomitoria_) is placed between the two. In others this part is wanting. Some Dipterous larvæ have two similar mandibles, but instead of being parallel, they are placed one above the other; others (_Musca domestica_ and _meridiana_) have but one such mandible, and some have no perceptible mandible of any kind. The mandibles of the larva of the crane-flies (_Tipula_), which are transverse and unguiform, do not act against each other, but against two other fixed, internally concave and externally convex, and dentated pieces[254].