460 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



an altitude of more than 20 feet and at a horizontal distance of over 

 100 feet from the moth — a performance that betters that of the most 

 sensitive microphones. 



DIRECTION 



Since differences in sound intensity are coded by the tympanic nerve 

 in at least four different ways, the horizontal bearing of a bat might be 

 derived from a comparison of the nerve responses to the same chirp in 

 the right and left ears. A difference in right and left responses might 

 be expected only if each ear had directional properties, that is, a lower 

 threshold to sounds coming from a particular direction relative to the 

 moth's axis. 



Directional sensitivity was measured in an open area where echoes 

 were minimal. A source of clicks of constant intensity was placed on 

 radii to the moth at 45° intervals. The source was moved in and 

 out on each radius until a standard tympanic nerve response was ob- 

 tained, and the distance from the moth noted. Horizontal distances 

 along eight radii were combined to make a polar plot of sensitivity 

 (Eoeder and Treat, 1961) . The plot showed that, although there was 

 little difference in sensitivity fore and aft, a click on the side nearest 

 the ear at about 90° relative to the moth's longitudinal axis was audible 

 at about twice the distance of a similarly placed click on the far side. 



This led to further field experiments in the presence of flying bats. 

 The tympanic nerve responses from both ears of a moth were recorded 

 simultaneously on separate tracks of a stereophonic magnetic tape. 

 The tape was subsequently replayed into a tw^o-channel oscilloscope 

 and the traces photographed (pi. 5) . In the upper record (A) the in- 

 crease in number of impulses in each succeeding train suggests the 

 approach of a bat. "Wlien the signals from right and left ears are 

 compared, it is evident that the greatest difference exists when the sig- 

 nal is faintest, the first response of the series occurring in one ear only. 

 Wlien both ears respond, the differential nature of the binaural re- 

 sponse can be seen first as a difference in the number of spikes gener- 

 ated in right and left ears, second in the differential spike frequency, 

 and third in the latency of the response, which is greater on that side 

 generating fewer spikes. It is also evident that, as the sound intensity 

 increases (presumably due to the approach of the bat) , the differential 

 becomes less until the responses of right and left ears become almost 

 identical. In another experiment, it was found that the tympanic 

 nerve response saturates, i.e., becomes maximal, when the sound inten- 

 sity is about 40 db (hundredfold) above threshold. From this it can 

 be concluded that the moth's nervous system receives information that 

 would enable it to determine whether a distant bat was to the right or 

 left, but if the bat was at close quarters this information would not be 



