The spinneret of the larvæ of Lepidoptera is evidently the homologue of the hypopharynx of insects of other orders. It will be seen that the homology of the different parts is identical, the common duct of the silk-glands opening at the end of the hypopharynx, which here forms a complete tube or proboscis extending beyond the end of the labium, in adaptation to its use as a spinning organ.
Walter refers to Burgess’s discovery of a hypopharynx in _Danais archippus_, remarking that this organ in the adult Eriocephalidæ (Fig. 76) exhibits a great similarity to the relations observable in the lower insects, adding:—
FIG. 77.— Labium of _Micropteryx anderschella_ seen from within (the labial palpi (_mx.′ p_) removed to their basal joint). Lettering as in Fig. 76.—After Walter. ]
“The furrow is here within coalesced with the inner side of the labium, and though I see in the entire structure of the head the inner edge of the ligula tube extended under the epipharynx as far as the mandible, I must also accept the fact that here also the hypopharynx extends to the mouth-opening as in all other sucking insects with a well-developed under-lip, viz. the Diptera and Hymenoptera.”
He has also discovered in Micropteryx a paired structure which he regards as the hypopharynx (Fig. 77). As he states:
FIG. 78.—Hypopharynx (_hph_) of Danais: _cl_, clypeus; _sd_, salivary duct; _m_, labial palp muscles; _fm_, frontal muscle; _ph_, pharynx; _cor_, cornea.—After Burgess. ]
“A portion of the inner surface of the tube-like ligula is covered by a furrow-like band which, close to the inner side, is coalesced with it, and in position, shape, as well as its appendages or teeth on the edge, may be regarded as nothing else than the hypopharynx.”
A hypopharynx is also present in the highest Lepidoptera, Burgess having detected it in _Danais archippus_. He states that the hypopharynx forms the floor of the pharyngeal cavity; “it is convex on each side of a median furrow (Fig. 78, _hph_) and somewhat resembles in shape the human breast. The convex areas are dotted over with little papillæ, which possibly may be taste-organs.”
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.