132 THE PHYSIOLOGY OF INSECT SENSES 



concentrations as the carbon chain length is increased. It is apparent 

 from Fig. 77 that this relation is not a continuous one. For each series 

 the curve shows a sharp break in the region of a definite chain-length 

 characteristic for each series. 



Taking a saturated straight-chain hydrocarbon as a starting-point, 

 one may, without altering appreciably the arrangement of the carbon 



ijj — 



-2.0 



-3.0 



20 



LOG NUMBER OF C- ATOMS 



Fig. 77. Rejection thresholds of glycols and alcohols by Phormia. (From 

 Dethier and Chadwick, 1948 b.) 



linkages, substitute various kinds of polar groups for one or more of 

 the hydrogen atoms. Whereas all such substitutions raise the molecular 

 weight and the boiling point, the effect on solubility is variable. The 

 different polar groups may be arranged in increasing order of solu- 

 bility as follows: Br < CI < CHg < CHO < C =- O < OH. The CH3 

 radical represents the unsubstituted compound. This series also repre- 

 sents in reverse order the relative stimulating efficiencies of these polar 

 groups. When the number of substitutions of the groups to the right of 



