CHEMORECEPTION 119 



Among the Diptera Liebermann (1926) has shown that the so-called 

 olfactory pits (Fig. 73) average 820 in dung-feeding species which de- 

 pend largely upon odour for locating their food, while in the flower- 

 visiting species, which depend more on vision, the average number is 

 494. Moreover, the pits are more numerous in males than in females. 

 This difference is presumed to be related to the fact that males employ 

 the olfactory sense in the search for females. 



In an individual there is clearly a relation between threshold and the 

 number of sensilla stimulated. The phenomenon was first investigated 



Fig. 73. Olfactory pit on the antenna of a dipterous insect. (From 



Liebermann, 1926.) 



with hygroreception. Pielou (1940) suggested that a threshold number 

 of receptors is required by Tenebrio molitor for a response. Detailed 

 confirmation was provided by the experiments of Roth and Willis 

 (1951 a), which showed that the percentage of response of a population 

 of Tribolium was closely correlated with the number of sensilla 

 basiconica remaining on each individual after surgical operation. The 

 change in threshold with unilateral antennectomy is illustrated by the 

 figures in Table 3 based on data of Roth and Willis (1951 b). For 

 olfaction the first quantitative data were those of Dethier (1952 b), 

 which showed that in Phormia regina there is a definite relation be- 

 tween the threshold and the number of receptors functioning. The 

 data suggest that for a response to occur a given number of molecules 

 I 



