134 THE PHYSIOLOGY OF INSECT SENSES 



also of prime importance in determining the solubility characteristics 

 of compounds of this type ; hence the same structural characteristics 

 that decrease water solubility likewise decrease threshold. 



There is then only one molecular property, for which data are 

 available, which brings all the data from the different series into a 

 single homogeneous system. This is water solubility (Fig. 78). The 

 order of stimulative efficiencies follows the inverse of the order of 

 water solubilities with fewer contradictions than appear in most of the 

 other comparisons attempted. Additional evidence that solubility is of 

 importance in this connexion has been presented by the work of 

 Dethier (1951a), which showed that the thresholds of alcohols are 

 altered as the alcohol is presented as an aqueous solution, a glycol 

 solution, or a mineral oil solution (Fig. 79). 



When threshold values are expressed as thermodynamic activities 

 rather than as moles the differences between successive homologues of 

 a series are not so marked, but a plot of the logarithm of these values 

 against the logarithm of activity coefficients (Fig. 74) does not produce 

 a straight line of the sort that one is accustomed to expect from 

 parallel experiments on narcosis (for a complete discussion consult 

 Ferguson, 1951 ; Brink and Posternak, 1948; Dethier, 1954 b). 



In spite of the generally good correspondence between low solu- 

 bility in water and high stimulating power, it seems likely, from the 

 data on oil-water partition coefficients, that plots of threshold values 

 against water solubihties would also yield a smaller slope for the 

 lower than for the higher range of compounds in each series if such an 

 analysis could be made. This type of relationship seems to have no 

 conterpart in any of the tabulated values for the physical properties, 

 and its consistent recurrence prompted Chadwick and Dethier (1949) 

 to consider the possibility that different forces may be of primary 

 importance in stimulation by the lower and higher members of each of 

 the types investigated. This amounts to postulating at least a two- 

 phase system for the limiting mechanism in contact chemoreception. 

 The hypothesis that small molecules gain access to the receptors in 

 part through an aqueous phase, whereas the larger aliphatic molecules 

 penetrate chiefly through (or accumulate in) a lipoid phase, would 

 appear to offer a basis for reconciling most of the contradictions en- 

 countered when it is attempted to fit the facts into a single-phase 

 system. 



Movement of the smaller molecules through an aqueous medium 

 should occur at rates related inversely to the molecular weight, which 

 would help to account for their being more stimulating than is antici- 



