CHEMORECEPTION 131 



labellar receptors ofPhormia (Dethier, 1955 a), and the receptors of a 

 number of other species (Frings, 1948) showed that the same general 

 relationships held. According to Frings, the data are in substantial 

 agreement with most of the data reported in the literature for chemical 

 stimulation of other animals by the series of cations employed, namely, 

 in decreasing order of thresholds: Li+ > Na+ > Mg++ > Ca++ = 

 Sr++>K+ = Cs+ = Rb+>NH4+>>>H+. As pointed out by 

 Frings, the order of effectiveness is also that of partition coefficients, 

 which parallels that of ionic mobilities (Osterhout et ai, 1934). The 

 arrangement for anions is less clear. The work of Frings (1946) estab- 

 lished the following order of thresholds: PO4 > Ac~ > SO4 = 

 CI- ^ Br- > I- > NO3- > > > OH-. 



The situation is actually much more complex than these generaliz- 

 ations imply. For example, the activity of divalent cations is anom- 

 alous in that they do not fit any of these series. Furthermore, the 

 behaviour of insects towards divalent ions is noticeably different from 

 their behaviour towards monovalent ions (cf. Hodgson, 1951). In 

 addition, there are species differences. It is possible that these may 

 reflect differences in the molecular structure of the receptors, as has 

 been proposed for mammals (Beidler, 1953, 1954, 1960; Beidler, Fish- 

 man, and Hardiman, 1955). 



Rejection Thresholds - Non-electrolytes 



The rejection thresholds of over two hundred aliphatic organic com- 

 pounds have been measured by employing flies that had been rendered 

 anosmic by removal of the antennae and labella (Dethier and Chad- 

 wick, 1947, 1948 b, 1950; Chadwick and Dethier, 1947, 1949). A 

 number of significant relationships were revealed by these data. 



Within any homologous series exceptionally high correlations were 

 found between the stimulating power of the compounds and such 

 properties as boiling point, molecular area, oil-water partition coeffi- 

 cients, molecular moments, vapour pressures, and activity coefficients. 

 Molecular weight, number of carbon atoms, and osmotic pressure 

 were eliminated from consideration in part by the use of isomers, 

 which proved usually to have different thresholds, and the correlation 

 with vapour pressures was inverse. Because of the paucity of data 

 relating to the various chemical and physical properties enumerated 

 above, it became convenient to plot stimulative efficiency against the 

 chain length of the molecule. Graphs so constructed (Fig. 77) show 

 clearly that for all series of homologous aliphatic compounds studied 

 the members of the series are rejected at logarithmically decreasing 



