In all insects this organ—whether forming a soft, tongue-like, anterior portion or fold of the labium, and “continuous with the lower wall of the pharynx,” or a hard, piercing, awl-like appendage (fleas and flies), or a long, slender, hairy or setose, trough-like structure like the “tongue” of the honey-bee—has a definite location at the end and on the upper side of the labium, and serves to receive at its base the external opening of the salivary duct.
The hypopharynx, as well shown in its lingua condition in Orthoptera, is continuous with and forms the anterior part or fold of the base of the coalesced second maxillæ. It does not seem to be paired, or to represent a pair of appendages.
Opinion regarding the homology of this unpaired piercing organ is by no means settled, and while there is a general agreement as to the nature of the paired mouth-parts, recent observers differ very much as to the morphology of the organ in question.
It is the langue or _lingua_ of Savigny (1816), the _ligula_ of Kirby and Spence (1828), the _langue ou languette_ (_lancette médiane du suçoir_) of Dugès (1832), the _lingua_ of Westwood (Class, ins., ii, p. 489, 1840), “the unpaired median piercing organ” (“the analogon of the epipharynx of Diptera”) of Karsten (1864), the “tongue” of Taschenberg (1880).
The name _hypopharynx_ was first proposed by Savigny in 1816, who, after naming the membranous plate which has for its base the upper side of the pharynx, the _epipharynx_, remarks: “Dans quelques genres, notamment dans les Eucères, le bord inférieur de ce même pharynx donne naissance à un autre appendice plus solide que le précédent, et qui s’emboîte avec lui. Je donnerai à ce dernier le nom de _langue_ ou d’_hypopharynx_. Voilà donc la bouche des Hyménoptères composée de quatre organes impaires, sans y comprendre la ganache ou le menton; savoir, la lèvre supérieure, l’épipharynx, l’hypopharynx, et la lèvre inférieure, et de deux organes paires, les mandibules et les mâchoires.”
As stated by Dimmock: “The hypopharynx is usually present in Diptera (according to Menzbier absent in Sargus), and contains a tube, opening by a channel on its upper surface; this channel extends back, more or less, from the tip, and is the outlet for the salivary secretion. The tip of the hypopharynx may be naked and used as a lance (Hæmatopota, according to Menzbier), or may be hairy (Musca). The upper side of the base of the hypopharynx is continuous with the lower wall of the pharynx; its under surface may entirely coalesce with the labium (Culex, male), may join the labium more or less, anterior to the month (Musca), or, if either mandibles or maxillæ are present, its base may join them (Culex, female).” (p. 43.)