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As a piercing organ the hypopharynx reaches its greatest development in the Siphonaptera and Diptera, where the chitinous parts are greatly hypertrophied, the fleshy tongue-like portion so developed in the mandibulate orders being greatly reduced. The chitinous parts are alike on each side of the median organ, being bilaterally symmetrical.

Fig. 79.—A, hypopharynx of Pulex canis: x, basal portion situated within the head; s. d, common duct of the four bladder-shaped salivary glands; s. d′, opening of the tubular salivary glands into the throat. B, end of the hypopharynx, showing the gutter-like structure and teeth at the end.—After Landois.

Fig. 80.—Beak of Vermipsylla: hyp, hypopharynx.—After Wagner.

In the fleas the hypopharynx is a large, slender, unpaired, long, chitinous trough, as long as the mandibles, and toothed at the end. Figures 79 and 80 show its relations to the other parts of the mouth; in Fig. 79, x, is seen where the salivary duct opens into the pharynx. Although this organ is not unanimously referred to the hypopharynx, yet from the description of Landois and others, it is evident that this structure does not correspond to the labrum or epipharynx, but belongs to or arises from the floor of the mouth, and, being in close relation to the labium, and also receiving the salivary duct, must be a true hypopharynx.