CHEMORECEPTOR MECHANISMS 



0.0 



0.4 



0.8 



LOG NUMBER OF C ATOMS 



Fig. 4. Comparison of relative effectiveness of homologous glycols in stimulating 

 tongue of man and tarsal receptors of the blowfly (Dethier, 1951b). 



general similarity to the results obtained with man in that the threshold de- 

 creases logarithmically with increase in chain length. For insects this is not an 

 even decrease. In the case of homologous alcohols the curve shows a break in 

 the region of the four and five carbon compound (Fig. 6). In series of aldehydes 

 and ketones the break is in the region of the three carbon homologue (Fig. 7). 

 In the glycol series there is a break in the region of the six carbon compound. 

 When large chain polymers are employed as stimuli, the break persists but in 

 the opposite direction, i.e., the increase of stimulating effect with increasing 

 chain length is less pronounced for the higher members than for the lower ones 

 (Fig. 6). In short, polymer mixtures do not react the same way as pure com- 

 pounds. 



It is possible to relate threshold rather accurately to chain length and to 

 make precise predictions on the basis of these relationships. But of all the col- 

 ligative properties of homologous compounds the only one which permits bring- 

 ing members of all series into one frame of reference is solubility. The threshold 

 is directly proportional to the water solubility (Fig. 8) (Dethier and Chadwick, 

 1950). Additional evidence that solubility is of importance in this connection 

 has been presented by the work of Dethier (1951a), which shows that the meas- 

 ured thresholds of alcohols are altered as the alcohol is presented as an aqueous 

 solution, a glycol solution, a mineral oil solution, (Fig. 9). When threshold values 



