122 THE PHYSIOLOGY OF INSECT SENSES 



as the chain length is increased until a cut-off point is reached at or near 

 Cii (Dethier and Yost, 1952). With aldehydes both rejection and 

 acceptance thresholds reveal similar relationships (Dethier, 1954 a). 



Mechanism of Olfaction 



The nature of the process whereby the odour molecule initiates de- 

 polarization in the olfactory receptor remains a mystery despite 

 numerous theories, of which those of Ehrensvard (1942), Davies 

 (1953a, 1953b), Mullins (1955 a, 1955b), and Davies and Taylor(1959) 

 are the more comprehensive. Analyses of relationships between thres- 

 hold of response and molecular characteristics have contributed much 

 of the groundwork for theorizing. On the basis of measurements made 

 with Phormia (Dethier and Yost, 1952; Dethier, 1954 a) and with man 

 (MulHns, 1955 a), it is clear that olfactory thresholds, stated as thermo- 

 dynamic activities, for members of homologous series of aliphatic 

 alcohols, aldehydes, and saturated hydrocarbons are nearly constant 



< 

 o 



-2 



12 3 4 



LOG foo (ZS^'C) 



Fig. 74. Comparison in terms of thermodynamic activity of the stimulating 

 effectiveness of the first eight normal alcohols acting in aqueous 

 solution (open circles) on tarsal chemoreceptors and as gases (solid 

 circles) on olfactory receptors of blowflies. In each case the value 

 represents a threshold of rejection. The vertical lines represent 2-575 

 standard errors for aqueous thresholds and 2 for vapours. (From 

 Dethier and Yost, 1952.) 



