SOUND RECEPTION 99 



mum sensitivity is in the range ( 1 5,000-60,000 c/s) corresponding to the 

 predominant frequencies in the cries of flying bats of the family 

 VespertiHonidae (Griffin, 1950, 1953). Actual tests in which electrical 

 recordings were made from tympanic organs in the presence of bat 

 cries prove beyond all doubt that the organs could be used for detect- 

 ing these predators (Roeder and Treat, 1957) (PI. 1). Behavioural 

 studies (Roeder and Treat, 1961 a, 1961 b) have shown that free-flying 

 moths take evasive action when they are stimulated by bat cries. This 

 is random action, at least when the bat is near, in that it bears no par- 

 ticular relation to the flight path of the bat. 



Like the tympanic organ of Orthoptera, the organs of moths are 

 directional receivers (Roeder and Treat, 1961 a). Localization is 

 theoretically possible with one, but is obviously more efficient with 

 two. At low sound intensities, that is, when the bat is not too near, 

 there is a differential response from the two organs, but as the intensity 

 increases, when the bat would be close, the differential nature of the 

 binaural response disappears (Roeder and Treat, 1961b) (Fig. 67). 



Another role of the tympanic organ may be concerned with navi- 

 gation during flight. It has been suggested (Hinton, 1955) that some 

 moths may orient themselves by echo-location. Although no experi- 

 mental proof exists, there is direct evidence that moths can detect the 

 sounds of a flying moth and presumably those of their own flying 

 (Roeder and Treat, 1957). This evidence suggests that moths might be 

 able to detect the reflection of their own flight sounds from nearby 

 objects, and hence to echo-locate. For this to succeed, however, numer- 

 ous obstacles would have to be overcome, not the least of which would 

 be the ability to distinguish between direct and reflected flight sounds. 



Function - Hemiptera 



Little is known about the tympanic organs of this Order. Males of the 

 waterboatman, Corixa, stridulate. Females respond to the chirping, 

 and other males chirp in chorus as long as the tympanic organs are 

 intact (Schaller, 1951). These are the only insects for which a sensitivity 

 to water-borne sound has been demonstrated. 



HAIRS AS SOUND RECEPTORS 



Many insects that lack tympanic organs are sensitive to sound waves in 

 air. As early as 1779, Bonnet recorded that caterpillars respond to 

 sound by thrashing about of the anterior portion of the body. Since 

 that time many naturalists have recorded this phenomenon, a response 

 brought about by convulsive contractions of the longitudinal muscles 



