The appendages forming the parts of the mouth are paired, and consist of the mandibles, the maxillae, and the labium, the pair in this latter part being combined to form a single body. The buccal appendages are frequently spoken of as gnathites. The gnathites are some, if not all, of them composed of apparently numerous parts, some of these being distinct sclerites, others membranous structures which may be either bare or pubescent—that is, covered with delicate short hair. In Insects the mouth functions in two quite different ways, by biting or by sucking. The Insects that bite are called Mandibulata, and those that suck Haustellata. In the mandibulate Insects the composition of the gnathites is readily comprehensible, so that in nearly the whole of the vast number of species of that type the corresponding parts can be recognised with something like certainty. This, however, is not the case with the sucking Insects; in them the parts of the mouth are very different indeed, so that in some cases morphologists are not agreed as to what parts really correspond with some of the structures of the Mandibulata. At present it will be sufficient for us to consider only the mandibulate mouth, leaving the various forms of sucking mouth to be discussed when we treat of the Orders of Haustellata in detail. The upper or anterior pair of gnathites is the mandibles, (Fig. 50, _g_). There is no part of the body that varies more than {95}does the mandible, even in the mandibulate Insects. It can scarcely be detected in some, while in others, as in the male stag-beetle, it may attain the length of the whole of the rest of the body; its form, too, varies as much as its size; most usually, however, the pair of mandibles are somewhat of the form of callipers, and are used for biting, cutting, holding, or crushing purposes. The mandibles are frequently armed with processes spoken of as teeth, but which must not be in any way confounded with the teeth of Vertebrates. The only Insects that possess an articulated tooth are the Passalidae, beetles armed with a rather large mandible bearing a single mobile tooth among others that are not so. Wood Mason and Chatin consider the mandibles to be, morphologically, jointed appendages, and the latter authority states that in the mandible of _Embia_ he has been able to distinguish the same elements as exist in the maxillae. In aculeate Hymenoptera the mandibles are used to a considerable extent for industrial purposes. [Illustration: FIG. 51.—Mandibles, maxillae, and labium of _Locusta viridissima_: A, mandibles; B, maxillae (lateral parts) and labium (middle parts) united: _a_, cardo; _b_, stipes; _c_, palpiger; _d_, max. palp.; _e_, lacinia; _f_, galea; _g_, submentum; _h_, mentum; _i_, palpiger; _k_, labial palpus; _l_, ligula; _m_, paraglossa (galea); _n_, lacinia; _o_, lingua.]