94 THE PHYSIOLOGY OF INSECT SENSES 



of approximately 10 metres to a chirping male. A female's path of 

 approach was nearly a straight line. Even with one tympanic organ 

 destroyed, orientation was possible, although the line of approach 

 now became more devious. These observations suggesting that the 

 tympanic organ has directional characteristics are supported by 

 experiments in which sensitivity was tested to a stimulus given 

 successively at different directions of incidence (Pumphrey, 1940). A 

 plot on polar co-ordinates of the sensitivity of an isolated tympanum 

 o^ Locust a as a function of the direction of incidence of the stimulus 

 illustrates the pattern of the directional characteristics (Fig. 64). This 

 finding is in agreement with the assumption that the tympanic organ 

 is a displacement rather than a pressure receiver (also Autrum, 1955 c) 

 Also, as these experiments revealed, the tympanic organ responds 

 equally well to 'push' and to 'pull'. 



Function - Lepidoptera 



Moths and butterflies also are known to be sensitive to sound (Stobbe, 

 1911 ; Turner, 1914; Turner and Schwarz, 1914; Eggers, 1925, 1926 a, 

 1928; Frings and Frings, 1956, 1957). Many of the sounds employed 

 for testing in early experiments were non-specific (e.g., squeaks of a 

 glass stopper twisted in a bottle), and the responses, too, were not 

 always specific (e.g., partial erection of the antennae, folding of the 

 wings, flexing of the antennae, running, or flying). It was difficult to 

 know what kind of sounds to employ as test stimuli for the simple 

 reason that no one knew what sounds moths responded to in nature. 

 One of the earliest speculations regarding the normal role of sound 

 perception in behaviour was the suggestion of White (1877) to the 

 effect that auditory receptors, in moths specifically, might assist in 

 detecting the approach of insectivorous bats. This idea gained plaus- 

 ibility from a number of field and laboratory observations (Schaller 

 andTimm, 1950; Webb, 1953; Treat, 1955), and was finally established 

 on a firm experimental basis by Roeder and Treat (1957, 1961 a, 

 1961 b). 



The principal sound receiver is the tympanic organ. The sensitivity 

 of this organ in noctuids to air-borne sounds was demonstrated by 

 Eggers (1925, 1926 a, 1928), first confirmed by the behavioural studies 

 of Schaller and Timm (1950) and of Treat (1955), and ultimately by 

 electrophysiological techniques. Other sound receptors must exist 

 because, as is true with Orthoptera, there is still a residual response to 

 intense sound after destruction of both tympanic organs (Stobbe, 

 1911; Eggers, 1925; Treat, 1955; Frings and Frings, 1957). Further- 



