CHEMORECEPTION 127 



thresholds in a population ofPhormia is not significantly different from 

 a normal distribution when plotted against the logarithm of concentra- 

 tion (Dcthier and Chadvvick, 1947). Where sufficient data have been 

 reported a similar relationship can be shown to exist with other insects 

 (e.g., Eger, 1937; Frings, 1946; von Frisch, 1935; Weis, 1930; 

 Hodgson, 1951). 



Threshold determinations have revealed that sensitivity of response 

 varies with the receptor field stimulated. In the horsefly Tabanus the 

 mean labellar threshold for sucrose is 0-02 IM; the tarsal threshold, 

 0-060M (Frings and O'Neal, 1946). In CallipJwra erythrocephala the 

 labella are generally more sensitive than the tarsi (Haslinger, 1935). In 

 C. vomitoria, on the other hand, the tarsi are sixteen times more 

 sensitive than the labellum to sucrose (Minnich, 1931), but lactose 

 elicits a response only when applied to the labellum (Minnich, 1929 b). 

 Musca domestica, Cochliomyia americana, and Phormia regina exhibit 

 lower rejection thresholds following oral stimulation than tarsal 

 stimulation (Deonier, 1938, 1939;Dethier, 1955a). Similar results have 

 been obtained WiihPieris (Verlaine, 1927). The antennae of honeybees 

 are more sensitive to solutions than the legs (Frings, 1944; Marshall, 

 1935 ; Minnich, 1932) ; the proboscis, more sensitive than the antennae 

 (Kunze, 1933). In the beetle Hydrous the labial palpi are sensitive to 

 hydrochloric acid and insensitive to sodium chloride and sugar, while 

 the maxillary palpi are sensitive to sodium chloride and sugar (Ritter, 

 1936). In the related species Laccophilus the antennae are more 

 sensitive than either pair of palpi to acids, salts, and alcohols (Hodgson, 

 1951). 



The aforementioned studies do not reveal whether the differences 

 in threshold reflect central phenomena, variations in the sensitivity of 

 the receptors, summation, or differences in sensitivity due merely to 

 the number of receptors stimulated. 



Differences between acceptance thresholds following unilateral and 

 bilateral tarsal stimulation had suggested that some relation does 

 exist between sensitivity and the number of receptors stimulated 

 (Imamura, 1938). An analysis of thresholds following stimulation of 

 one tarus versus two tarsi in Phormia showed that the bilateral thres- 

 hold for sugar is lower than the unilateral threshold (Dethier, 1953 b). 

 This difference could be due, however, either to summation or to a 

 simple statistical bias. In the analogous visual case where comparisons 

 have been made between monocular and binocular vision, Pirenne 

 (1943) and Barany (1946 a, 1946 b) pointed out that the experimental 

 procedure by its very nature assures that the two eyes will see more 



