28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



depression between them into a deep heart-shaped cavity. The bib 

 dependent from the lower lip of the mouth, however, has now 

 assumed a nearly horizontal position with its points resting on the 

 hypopharyngeal surface of the prementum, and thus forms a ramp 

 from the floor of the food channel to the mouth, bridging the cavity 

 behind it. 



The cavity temporarily formed behind the suboral fold in the bee 

 by the infolding of the adoral hypopharyngeal surface (fig. 10 A, C, 

 IbS) evidently corresponds with the permanently differentiated 

 infrabuccal sac of certain other Hymenoptera. In Vespidae the sac 

 is a well-developed structural feature of the preoral cavity, completely 

 closed except for a narrow, transverse slitlike opening (fig. 9 D, IhS) 

 beneath a broad suboral fold {bib) corresponding with the bib of the 

 honey bee. 



The preoral feeding mechanism of the wasps, as described by 

 Duncan (1939) in Vespula, is similar to that of the bee, except for 

 the more highly developed infrabuccal sac. The hypopharyngeal rods 

 are shorter and thicker in Vespula (fig. 9 D, E, s) than in Apis, and 

 are termed by Duncan the "labral tractors" because when the 

 proboscis is extended they pull down on the long basal angles of the 

 labrum (E, Lm) and thus open the mouth. The labrum of the wasp, 

 however, is entirely detached from the edge of the clypeus {Clp) and 

 is retracted beneath the latter. The epipharynx of the wasp, accord- 

 ing to Duncan, has no muscles ; in the bee it is provided with a group 

 of muscle fibers arising on the clypeus (fig. 10 C, 2^). A short branch 

 of each hypopharyngeal rod in Vespula (fig. 9 D, E, x), the "trac- 

 toral suspensor" of Duncan, goes laterally to the base of the mandible, 

 and is suggestive of being the mandibular arm of the lateral hypo- 

 pharyngeal sclerite of generalized insects, on which the hypopharyngeal 

 muscle of the mandible, when present, is attached. 



Just within the mouth on the floor, or posterior wall, of the entrance 

 to the sucking pump is the broad oral plate (fig. 10 A, opl), which is 

 commonly known as the "pharyngeal plate." There can be little 

 doubt, however, that the plate in question is a hypopharyngeal struc- 

 ture, since from its proximal angles two long arms (A, B, C, 3;) 

 extend upward and anteriorly in the lateral walls of the pump and 

 give attachment to muscles arising on the frons (B, C, ^2, jj). 

 Similar though shorter rods giving insertion to frontal muscles are 

 present in most insects, and usually are prolongations from hypo- 

 pharyngeal sclerites; the rods, together with their muscles, in fact, 

 constitute the adductor apparatus of the hypopharynx. In some cases 

 the pharyngeal rods are processes of a plate on the base of the 



