30 THE PHYSIOLOGY OF INSECT SENSES 



frequency because the interval between impulses is equal to the re- 

 fractory period of the neuron there is a direct relation between fre- 

 quency and the amplitude of the generator potential. These relations 

 hold for the insect mechanoreceptor (Figs. 21, 22). 



The most striking feature of insect mechanoreceptors is the change 

 in size of the impulse as the receptor potential changes (Figs. 23, 24, 



o 

 o 



o 



Threshold Receptor Potential for Appearance of impulses MV 



Fig. 19. The threshold receptor potential of a mechanoreceptor on the 

 grasshopper claw which is accompanied by an impulse is plotted for a 

 series of test stimuli. In the largest number of trials the threshold was 

 2-5 MV with the spread as shown. (Redrawn from Wolbarsht, 1960.) 



25) (Wolbarsht and Dethier, 1958; Wolbarsht, 1960). Wolbarsht 

 (1960), after analysing the electrical events occurring in the mechano- 

 receptor, concluded that: the mechanical stimulus effects a change 

 in the membrane resistance at the receptor site; the receptor potential 

 (which is undoubtedly the generator potential) is the difference 

 between the potential across the membrane at the receptor site and the 

 general polarization of the cell; the impulses are generated proximad 



