102 THE PHYSIOLOGY OF INSECT SENSES 



'spontaneous' activity recorded from nerves in silent surroundings 

 may represent random movement of the dendrites by Brownian 

 agitation of the molecules in their vicinity. Conditions are similar for 

 the cricket and the cockroach. 



In contrast to the tympanic organ, the cereal hairs exhibit a number 

 of characteristics seen in recordings from the mammalian cochlear 

 nerve. One of these is synchronization. At low stimulus frequencies 

 the action potentials in the nerve are exactly synchronized with the 

 stimulus. At the low end of the frequency range the nerve may be 

 synchronized or show what Pumphrey and Rawdon-Smith (1936 a, 

 1936 b, 1936 c) term 'frequency doubling'. At high frequencies it may 

 show frequency halving or quartering. Thus, at a stimulus frequency 

 of 400 c/s the nerve may discharge at 400 c/s or at 800 c/s, and at a 

 stimulus frequency of 400 c/s, fire at 300 or 150 c/s. In the 400 c/s 

 range of stimulation the nerve that is firing in synchrony may begin 

 halving if the intensity of the stimulus is reduced by about 10 db. 

 Pumphrey and Rawdon-Smith (1936 c) suggested that these phe- 

 nomena may exemplify the condition known as 'alternation'. In the 

 mammalian Vlllth nerve the synchrony in the whole nerve at high 

 stimulus frequencies is believed to result from each fibre firing to 

 alternate sound waves and the different fibre groups being 180 degrees 

 out of phase. In the cereal nerve the alteration is diff'erent in that all 

 fibres fire to every other sound wave but all are in phase, that is, the 

 whole nerve alternates, hence, the response frequency is halved or 

 quartered. It was suggested, however, that true alternation of the 

 mammalian type also occurs, at least during the first fraction of a 

 second of response, when the nerve momentarily fires at 600-800 c/s. 

 At higher stimulus frequencies the nerve responds asynchronously. 



Another resemblance to the mammahan Vlllth nerve is the occur- 

 rence of equilibration, that is, a decline with time in the amplitude of 

 massed spikes in the nerve. This phenomenon is interpreted as a 

 reduction in the number of impulses at each sound wave with time as 

 the result of the lengthening of the relative refractory period of each 

 fibre. 



Electrophysiological studies of the cereal sensilla of a number of 

 Acrididae are in general agreement with the results just described; 

 however, no clear evidence of equilibration or frequency halving or 

 doubling was obtained (Haskell, 1956 a, 1956 b). 



Hairs on other parts of the body, especially of Orthoptera, are 

 believed also to be receptors sensitive to air-borne sound waves, but 

 the evidence is not always convincing, because it is derived primarily 



