128 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



for their presumed function 



l^-.k 



-sc 



C — 



to a degree of excitability which leads to the consummation of 

 the sexual act. Whether the placoid sensillae are actually the 

 receptors involved has not been absolutely proved. Structurally, 

 rhinaria appear to be well adapted for this function, since they 

 are invested by an extremely thin cuticle and are innervated by a 

 multinucleate sense-cell {vide Chrystal, 1930), but in the hive bee, 

 for example, the placoid sensill?e appear to be but poorly adapted 



As Snodgrass points out, the 

 plates themselves are about 1-5 /x 

 in thickness, but the peripheral 

 membrane of the surrounding 

 groove is not more than 0-5 /x in 

 thickness. It would seem, there- 

 fore, that it would be necessary 

 to assume that a liquid filters 

 through the thin membrane of the 

 groove and spreads itself over the 

 surface of the plate. There is as 

 yet, hov»'ever, no proof that this 

 actually takes place. ^Iclndoo 

 regards the campaniform sensillse 

 as being the true olfactory 

 receptors. He termed these 

 structures " olfactory pores " for 

 the reason that he claims that the 

 covering membrane of these 

 minute dome-like structures is perforated by a central pore which 

 thus allows the process of the sense-cell concerned to come in 

 direct contact with odoriferous particles. Notwithstanding the 

 large amount of histological study that has been devoted to these 

 structures, no other observer has detected the presence of the 

 central pore which he claimed to be evident. The most recent 

 workers, including Sihler (1924) and Newton (1931), state that the 

 covering membrane of the sensilla is continuous and devoid of any 

 kind of pore. It seems probable that what Mclndoo described 

 is merely an optical effect caused by a thin central area of cuticle 

 wherein the apex of the scolopala is lodged. 



niA 



Fig. 56. Campaniform sensilla from 

 the cercus of Blatta orientalis. c, 

 Chitogenous cell ; d, dome-like 

 cuticiilar covering ; h, hypo- 

 dermis ; n, neurilemma ; iw, 

 nerve fibre ; s, scolopala ; sc, 

 sense-cell ; v, vacuole ; (integu- 

 ment deep black). (From Sihler.) 



