146 THE PHYSIOLOGY OF INSECT SENSES 



impulses. Each of these functions is currently the subject of intensive 

 study. 



The suggestion was made by Dethier (1956) that chemicals de- 

 polarize the membrane of the distal process of the neuron at the tip of 

 the hair, that this depolarization travels down the nerve fibre to the 

 cell body at the base of the hair, and that the action potential is gen- 

 erated in this region. Studies of the effects of temperature on thresholds 

 and on frequency of impulse discharge lent some support to this idea. 

 When the temperature of the tip of the hair, and hence the receptor 

 surface, was altered in the range 2°-41° C, no change in behavioural 

 threshold occurred (Dethier, 1956), nor was there any change in the 

 frequency of impulses (Hodgson, 1956 b; Dethier and Arab, 1958). 

 When the temperature of the whole preparation, and hence the cell 

 bodies, was altered, however, marked changes in frequency occurred 

 (Hodgson and Roeder, 1956; Hodgson, 1956 b). According to 

 Hodgson (1956 b), the fibres in some hairs increased their frequency 

 with warming while others decreased with warming. 



The most conclusive evidence bearing on the matter came, as might 

 be expected, from electrophysiological work. Wolbarsht (1958), seek- 

 ing evidence for a slow potential associated with stimulation, dis- 

 covered when he placed one electrode on the tip of a hair and the other 

 in the crushed head or labellum that there was a resting potential of the 

 order of 60 mV. Strangely enough, however, this remained unaltered 

 when the hair was stimulated. He concluded that the potential repre- 

 sented a difference across the basement membrane which separates 

 the general body cavity and the area containing the neurons, associ- 

 ated cells, and hypodermis. Later, working exclusively with the 

 mechanoreceptor component of the chemoreceptive hair (and with 

 simple mechanoreceptors), he recorded a graded slow potential which 

 occurred when the hair was stimulated by bending. It was always an 

 increase in negativity at the recording electrode. It varied directly with 

 the magnitude of the stimulus and showed no overshoot when return- 

 ing to the baseline. This is a receptor potential in that it occurs prior 

 to the initiation of any impulses, varies smoothly, and must attain a 

 critical level before any impulses are generated. It is also either the 

 generator potential itself or is associated with it. Evidence was pro- 

 duced that suggests that the receptor potential originates at a site that 

 is not invaded by the propagated impulses and that this site is distad 

 of the site of impulse generation. The geometry of the hair and the 

 mechanoreceptor indicates that this must be in the distal process of the 

 cell. 



