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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. y-j 



esses, which Hes in a vacuole (Fac) within the enveloping cell. 

 Cross-sections show that there is probably only one enveloping cell 

 to each sensillum (C, ECl), though both Vogel (1923) and the 

 present writer in a former work (1925) have shown two. The sen- 

 silla lie so close together that, in sections cut vertical to the surface, 

 it is impossible to distinguish the cells of neighboring organs. The 

 sense cells (A, SCls) of all the sensilla of the antennal organs of 

 the honeybee form one continuous mass of sensory cells beneath the 

 normal hypodermis, nearly surrounding the lumen of the antenna, 



/ECI 

 'Vac 



Fig. 24. — Diagrammatic structure of a sensillum placodeum of Apis 

 mcUifcra. 



A, vertical longitudinal section. B, cross-section just beneath plate. 

 C, cross-section near base of enveloping cell. 



being absent only along the outer side where there are no sense 

 organs. 



It would be useless to review here all the opinions that have been 

 held concerning the function of the sensilla placodea, since there is 

 little direct evidence connecting any specific sense with these organs. 

 Some writers have regarded them as having an auditory function, 

 others have sought to explain them as organs for perceiving air 

 pressure, and still others have believed that they are organs of smell. 

 Since it seems now pretty well attested that the principal seat of the 

 olfactory sense in bees is located on the antennse, the most character- 

 istic organs of the antennae might be supposed to be the organs of 

 smell. Suspicion, therefore, falls upon the plate organs. Yet, these 



