Summary
If the primary function of ultrasound-sensitive ears in Lepidoptera is to detect the echolocation calls of insectivorous bats, diurnal species should exhibit signs of ultrasonic deafness. To test this hypothesis we recorded the 24-h (diel) flight activity of a sample of Neotropical butterflies and moths, including members of the Dioptinae, a reportedly diurnal moth subfamily. All of the butterflies examined were exclusively diurnal and we suggest that these insects are day-locked because they lack the ultrasound-sensitive ears that permit nocturnal taxa to co-exist with bats. Nondioptine moths possess sensitive, bat-tuned ears and exhibit levels of nocturnal flight ranging from complete to mixed day/night activity. Dioptine moths are significantly less nocturnal and significantly deafer than nondioptines and exhibit flight activity patterns ranging from completely diurnal to completely nocturnal. We suggest that diurnality in the Dioptinae is an apomorphic trait that has resulted in a loss of auditory sensitivity in some species but that others may retain functional ears depending on their levels of nocturnality and consequent exposure to bats.