The epipharynx is so intimately associated with the elongated labium of certain Diptera, that, with Dr. Dimmock, we may refer to the double organ as the labrum-epipharynx; and where, as in the lepidopterous _Micropteryx semipurpurella_, described and figured by Walter, and the Panorpidæ (Panorpa and Boreus), the labrum seems pieced out with a thin, pale membranous fold which appears to be an extension of the epipharynx, building up the dorsal end of the labrum, this term is a convenient one to use.
In the lower orders of truly mandibulate insects, from the Thysanura to the Coleoptera, excluding those which suck in liquid food, such as the Diptera, Lepidoptera, and Hymenoptera, and the Mecoptera (Panorpidæ) with their elongated head and feeble, small mandibles, the epipharynx forms a simple membranous palatal lining of the clypeus and labrum. In such insects there is no soft projecting or pendant portion, fitted to close the throat or to complete a partially tubular arrangement of the first and second maxillæ.
In all the mandibulate insects, then, the epipharynx forms simply the under surface or pharyngeal lining of the clypeus and labrum, the surface being uniformly moderately convex, and corresponding in extent to that of the clypeus and labrum, posteriorly merging into the palatal wall of the pharynx; the armature of peculiar gathering-hairs sometimes spreading over its base, being continuous with those lining the mouth and beginning of the œsophagus. The suture separating the labrum from the clypeus does not involve the epipharynx, though since certain gustatory fields lie under the front edge of the clypeus, as well as labrum, one may in describing them refer to certain fields or groups of cups or pits as occupying a labral or clypeal region or position.
The lack of traces of a suture in the epipharynx corresponding to the labral suture above, suggests that the labrum does not represent a pair of coalesced appendages, and that it, with the clypeus, simply forms the solid cuticular roof of the mouth.
The only soft structures seen between the epipharynx and labrum, besides the nerves of special sense, are the elevator muscles of the labrum, and two tracheæ, one on each side.