Persistence and regression of hearing in the exclusively diurnal moths, Trichodezia albovittata (Geometridae) and Lycomorpha pholus (Arctiidae)

Auditory sensitivities and ultrasound avoidance behaviour of two exclusively diurnal moths were examined to test the prediction that total isolation from the predatory effects of echolocating bats will result in the regression of these sensory systems and/or the defences they evoke.The silent geometrid, Trichodezia albovittata, possessed large ears with auditory neural thresholds similar to or better than those of a sympatric, exclusively nocturnal geometrid moth. Trichodezia albovittata readily responded with evasive flight to ultrasound and it is suggested that if this moth has become completely isolated from bats its ears are functionally vestigial, at least in the population studied here. In contrast, while the sound‐producing arctiid, Lycomorpha pholus, had low auditory sensitivity based on neural thresholds, it still responded with flight changes to ultrasound. It did not, however, produce sounds when stimulated ultrasonically. It is suggested that the ears of this moth are functionally vestigial for bat‐detection purposes but may be used for short‐distance social communication.