Diel flight periodicity and the evolution of auditory defences in the macrolepidoptera

We measured the 24-h flight activity patterns of 84 species of Nearctic Lepidoptera representing 12 ultrasound-earless and seven ultrasound-eared families to examine the evolution of the diel flight periodicities (DFPs) and auditory defences of these insects. Most species tested showed mixed DFPs (flight during day and night hours) with few being exclusively nocturnal. With the exception of one geometrid moth and one arctiid moth, only the butterflies (Papilionoidea+Hesperioidea) were exclusively diurnal, supporting the hypothesis that this trait has resulted from their lack of ultrasound-sensitive ears, structures that in most other Macrolepidoptera provide auditory defence against aerially foraging, insectivorous bats. Eared species showed the highest tendency to fly during both day and night, a trait we suggest results from their possession of both visual and auditory defences. Earless moths were almost exclusively nocturnal and may be constrained to the night because of their size and tendency for ground-hugging flight, which may provide protection against bats, but would be liabilities against diurnal, visually hunting predators. Ultrasound sensitivity not only appears to protect eared moths from aerial predators (bats) but also protects them from terrestrial predators by allowing the moths to remain in the air during the night, and, we suggest, is responsible for the success of this group of Macrolepidoptera.