OLFACTORY SENSILL^ 127 



membranes may be permeable to liquids, and the existence of 

 the merest film bathing the exterior of a sense-organ would be 

 extremely diffieult to deny or affirm. The alternative hypothesis 

 is that the membrane is of a texture permeable to many types of 

 odoriferous particles, which pass into solution immediately upon 

 coming into contact with the vacuolar fluid surrounding the 

 nerve-ending of the sense-cell within. 



In the study of chemoreceptors we are met with a baffling 

 variety of sensillae often widely distributed over the insect body. 

 As Wheeler remarks (1928), the confusion is increased by the 

 difference of opinion prevailing in regard to their structure in 

 relation to their possible functions. It grades from sensillae which 

 may be either tactile or olfactory, through a variety of presumably 

 olfactory and gustatory to the Hicksian or campaniform sensillae, 

 which have been variously interpreted as organs of pressure, 

 temperature, humidity, or vibration. 



The usual course has been to determine by suitable experiments 

 which appendages or regions of the body afford evidence of 

 response to olfactory stimuli. The parts in question are then 

 subjected to histological examination, and those sensillae present 

 which appear structurally adapted for the reception of the 

 stimulus in question are regarded as olfactory receptors. Thus 

 placoid sensillae are abundant upon the antennae of certain 

 Homoptera, many Coleoptera and most Hymenoptera. In many 

 of the Parasitic Hymenoptera they are elongated structures 

 sometimes termed rhinaria ; in Aphididae they are commonly 

 circular in form, while in the hive bee they are more ovoid. As a 

 general rule placoid sensillae are more abundant in the male than 

 in the female ; in aphides they are most numerous in the alate 

 forms. In the hive bee, for example, Vogel computed that in the 

 drone each antenna bears about 30,^00 of these sensillae ; in the 

 worker about 6,000 ; and in the queen between 2,000 and 3,000. 

 The general conclusion is that the greater number of these organs in 

 the male is correlated with the more highly developed olfactory 

 powers in that sex, which come into play in seeking the female. 

 Furthermore, sexual odour emanated by the female, acting upon 

 the great number of receptors in the male, stimulates the male 



