Directional sensitivity of the ears of noctuid moths.

Moths of several families, notably the Noctuidae, Arctiidae and Geometridae, have tympanic organs sensitive to pulsed ultrasound (Eggers, 1919; Haskell & Belton, 1956; Roeder & Treat, 1957). It has long been suspected that moths could hear the cries of approaching bats which prey upon them (White, 1877) and a variety of behavioural responses to natural and artificial acoustic stimuli have been reported (Eggers, 1926; Schaller & Timm, 1950; Treat, 1955). An attempt has been made to track and to describe the reactions of free-flying moths in the field to a fixed source of artificial ultrasonic pulses (Roeder, 1962), and to relate various forms of evasive behaviour to the pattern of afferent nerve impulses elicited by ultrasound in the pair of acoustic sense cells (A cells) of the noctuid tympanic organ (Roeder, 1964). The behavioural responses of moths are conspicuously different to pulses of high and low intensities. In the presence of high-intensity pulses the moth's responses take the form of diving and looping, but since these responses generally bring the moth to the ground and show no clear directional relation to the source of ultrasound they will not be considered further. However, it was consistently noted (Roeder, 1962, 1964) that moths flying at greater distances from the sound source, and at various altitudes with respect to it, would turn and fly (with some preliminary 'hunting') directly away from the source. This 'turning-away' was observed only if the moth encountered a series of low-intensity ultrasonic pulses and it is likely that at such intensities only one nerve cell in each ear is active. Further, it implies that moths with this sensory equipment can extract directional information from sound waves in all planes. A pair of symmetrically placed ears containing only one responding sense cell per ear can convey information about the direction of a source of sound pulses if two conditions are met. First, the receptor cell must convey, in the pattern of its afferent fibre discharge, differences in the intensity of stimulation. In the present case ' turning-away' occurs only at low sound levels. Therefore, the intensity discrimination of the most sensitive tympanic sense cell (Ai) must be greatest at sound intensities not much above its threshold. The acoustic sense cells of noctuid moths are indeed