148 THE PHYSIOLOGY OF INSECT SENSES 



receptors are conceived of as being recorded by electrotonic spread up 

 through the contents of the hair, which would be analogous to an 

 external electrode opposite a part of the cell membrane that is not 

 involved in the propagated impulse. This conception is supported by 

 the results of polarization experiments. Anodal polarization produces 

 stimulation; cathodal stimulation produces depression, as has been 

 seen with intracellular electrodes in the eye of Limulus. 



Arab (1958) found that behavioural responses were evoked by 

 cathodal stimulation and would not occur with anodal stimulation. 

 Wolbarsht (1958) was able to reconcile this apparent discrepancy. He 

 showed that the behavioural responses observed by Arab were due to 

 the action of the current on the neurons at the bases of hairs adjacent 

 to the one on which the electrodes were placed. In this case the polariz- 

 ing current was extracellular; hence, cathodal stimulation was 

 excitatory and anodal stimulation, the reverse. 



Working with the ^yLucilia caeser and the butterfly Vanessa indica, 

 Morita and his co-workers attempted to pinpoint the site of spike 

 generation (Tateda and Morita, 1959; Morita and Takeda, 1959; 

 Morita, 1959; Morita and Yamashita, 1959 a, 1959 b). First, they 

 showed that spikes were never recorded from the tip of a hair when it 

 was severed from its base. They then confirmed the findings of 

 Wolbarsht (1958) that anodal stimulation elicited impulses and 

 cathodal current blocked them. They found also, as Wolbarsht had, 

 that anodal stimulation of one hair blocked activity in the neighbour- 

 ing hair, but they did not do a similar experiment with cathodal 

 stimulation as Wolbarsht had. 



In an attempt to explain how spikes can be recorded from the tip of 

 the hair, Morita (1959) cooled first the middle and then the base of the 

 hair, recorded in each case from the tip, and analysed the shape of the 

 spike. According to his records, cooHng the middle affected pre- 

 dominantly the falHng phase of the spike, while cooling at the base 

 aff'ected both the rising and falHng phases. On the basis of this evidence 

 he claimed that spikes originate at the base of the hair and fire in both 

 directions, that is, back along the distal process to the tip, as well as 

 centripetally along the axon to the central nervous system. If this 

 condition occurs, it would mean that spikes invade the area of the 

 generator potential contrary to the situation found in mechano- 

 receptors. 



Morita and Yamashita (1959 b) recorded simultaneously from the 

 tip and the base and found that the spike at the tip is generally larger 

 than the spike recorded from a hole in the side wall. They observed that 



