Phonoreception 497 



It merely seems probable, on the basis of present evidence, that frequency 

 discrimination is of a low quality. 



The next question is whether or not insects are capable of a high quality 

 of discrimination of amplitude modulation. It has been proposed by Pumphrey 

 and Rawdon-Smith •'♦• •'•''• ■^'' that they are capable of such discrimination and 

 that they are also sensitive to small changes in the frequency of the modu- 

 lating frequency. It was demonstrated experimentally that amplitude modu- 

 lation of a sound wave could produce a temporal pattern of impulses in the 

 insect auditory nerve. Pumphrey and Rawdon-Smith propose that sound 

 quahty discrimination in insects is more a discrimination between amplitude 

 modulation frequencies than between frequencies of the sound waves them- 

 selves. The insect tympanic organ would therefore be more nearly com- 

 parable to the audio detector in a radio set than to a harmonic analyzer. 

 The human ear is constructed so that it is highly sensitive to changes in 

 the modulated frequency, i.e., it is an excellent harmonic analyzer, but we 

 are relatively insensitive to changes in the modulation frequency. 



If Pumphrey and Rawdon-Smith's theory is true, then the seemingly con- 

 tradictory experiments on insect hearing can be explained. For instance, in 

 Regen's experiments the response of the female crickets might be explained 

 on an amplitude modulation basis, and so might the unresponsiveness of 

 experienced male katydid concert singers who failed to answer unmodulated 

 artificial partners. Furthermore, the more common insect sound-producing 

 methods, e.g., the rubbing together of legs and wings, are probably more 

 adaptable to the production of amplitude modulated sounds than are the 

 air column devices of vertebrates. This theory of Pumphrey and Rawdon- 

 Smith is one which warrants considerable investigation. 



Sound Localization. Regen ■^'^ observed that unmated female crickets 

 oriented toward a chirping male at a distance of ten meters or more, and 

 that they tended to move toward the male along a straight line. Females 

 deprived of one tympanic organ reached the male over a more devious path. 

 It is evident that the female was capable of locating the sound. 



In man, localization is determined by intensity and phase differences 

 in the two ears produced as a result of their separation and of the shadow 

 effect of the head. In insects the influence of these two factors is very much 

 less, and they are probably insignificant. However, the structure of the 

 tympanum indicates that, especially when exposed on the body surface, it 

 may serve directly as a localizer. Pumphrey and Rawdon-Smith ^^ demon- 

 strated this for Locusta. They used a monophasic transient sound wave as 

 a stimulus and determined the intensity necessary to stimulate the isolated 

 tympanic organ as a function of the angle of incidence. The threshold was 

 determined bv electrical recording from the nerve. The tympanic organ has 

 very definite directional characteristics which, in general, are comparable to 

 those of a "ribbon" or "velocity" microphone. Since the stimulus used was 

 a monophasic sound wave it was possible to compare the response of the 

 tympanum to a "push" or "pull." No differences were found either in the 

 threshold or in the latent period when the stimulus was reversed. This is in 

 striking contrast to the vertebrate cochlea. 



Hair Sensilla as Phonoreceptors. It is well known from the work of 



