The head is most variable in size and form; as a part of its surface is occupied by the eyes and as these organs differ in shape, extent, and position to a surprising degree, it is not a matter for astonishment that it is almost impossible to agree as to terms for the areas of the head. Of the sclerites of the head itself there are only three that are sufficiently constant and definite to be worthy of description here. These are the clypeus, the epicranium, and the gula. The clypeus is situate on the upper surface of the head-capsule, in front; it bears the labrum which may be briefly described as a sort of flap forming an upper lip. The labrum is usually possessed of some amount of mobility. The clypeus itself is excessively variable in size and form, and sometimes cannot be delimited owing to the obliteration of the suture of connexion with the more posterior part of the head; it is rarely or never a paired piece. Occasionally there is a more or less distinct piece interposed between the clypeus and the labrum, and which is the source of considerable difficulty, as it may be taken for the clypeus. Some authors call the clypeus the epistome, but it is better to use this latter term for the purpose of indicating the part that is immediately behind the labrum, whether that part be the clypeus, or some other sclerite; the {93}term is very convenient in those cases where the structure cannot be, or has not been, satisfactorily determined morphologically. In Figure 50 the parts usually visible on the anterior aspect of the head and its appendages are shown so far as these latter can be seen when the mouth is closed; in the case of the Insect here represented the bases of the mandibles are clearly seen (_g_), while their apical portions are entirely covered by the labrum, just below the lower margin of which the tips of the maxillae are seen, looking as if they were the continuations of the mandibles. The labrum is a somewhat perplexing piece, morphologists being not yet agreed as to its nature; it is usually placed quite on the front of the head, and varies extremely in form; it is nearly always a single or unpaired piece; the French morphologist Chatin considers that it is really a paired structure. [Illustration: FIG. 50.—Front view of head of field-cricket (_Gryllus_): _a_, epicranium; _b_, compound eye; _c_, antenna; _d_, post-: _e_, ante-clypeus; _f_, labrum; _g_, base of mandible; _h_, maxillary palpus; _i_, labial palpus; _k_, apex of maxilla.]