SOUND RECEPTION 93 



latter hypothesis Haskell (1956 b) played the identical stimulus again 

 and again to a tympanic organ and recorded the pattern of discharge 

 from the tympanic nerve. He found that the resultant volleys of spikes, 

 although synchronous with the pulse frequency of the stimulus, were 

 variable in themselves and that this variability bore no simple relation- 

 ship to any quality of the stimulus. On the basis of this evidence he 



Fig. 64. Sensitivity of an isolated tympanic organ of Locusta plotted on 

 polar co-ordinates as a function of direction of incidence of the test 

 stimulus. Sensitivity (log reciprocal of threshold amplitude) is plotted 

 radially and the minimum sensitivity is arbitrarily taken to be zero. 

 The line 0-180 degrees lies in the sagittal plane of the animal, and 

 for angles of positive or negative sign the test stimulus is incident on 

 the external or internal aspect of the tympanic organ respectively. 

 (Redrawn from Pumphrey, 1940.) 



concluded that the characteristic of stridulation that permits inter- 

 specific recognition is the pulse repetition of the songs. Many ques- 

 tions about sound recognition remain to be answered, however, and 

 Haskell's (1956 b) discussion should be consulted. 



One feature of the early field experiments that turned up repeatedly 

 was the ability of the insects to locahze sound. Regen (1924), for 

 example, showed that unmated females could orient from a distance 



