92 



DIETRICH SCHNEIDER 



above a concentration of lOi'V/zg/ml solvent. The saturation level of the 

 motor response, but not necessarily of all the receptors involved, is reached. 



Similar work was done on the gypsy moth by B. C. Block (1960 and un- 

 published). This investigator used a different method, mounting a filter 

 paper piece contaminated with known amounts of the Portlietria attractant 

 in front of a rack with 30 male moths. With an air current directed toward 

 the rack, the males reacted typically to lure concentrations of above 

 threshold. 



The results for Bombyx and Portlietria, although worked out with dif- 

 ferent methods, have a similar stimulus-response course in common. 

 Beginning from a certain scent concentration, the reaction curve rises as a 

 straight line when the response is plotted linearly and the lure concentration 

 logarithmically (Fig. 5). The upper " saturation " plateau is reached 

 after increasing the stimulus intensity between 100 and 1000 times above 



W0% 



50 



Fig. 5. The behavioural stimulus-response curve o^ Portlietria dispar males to 

 the sexual odour of its own female and of Bombyx mori males to the synthetic 

 Bombykol. Ordinate : per cent of males reacting. Abscissa : Amount of 

 lure substance on a stimulus source (from Block, Boeckh and Schneider, un- 

 published). 



threshold. The most remarkable difference between these two behavioral 

 investigations is the position of the absolute threshold. Block found that 

 gypsy moth males do not react below 10" V/^g of substance on filter paper. 

 This differs from Bombyx, where less than 10"i V/^g of Bombykol was on the 

 glass rod tip at threshold and cannot be explained by methodology. We 

 must envisage a difference in the absolute reaction threshold of the two 

 species. 



In an earlier electrophysiological study, the stimulus-response function 

 of the whole antenna of Bombyx males was found to increase as the 

 logarithm of the stimulus concentration (Schneider, 1962). This work 



