Ultrasonic hearing is common in moths, which rely on it for defence and communication, but it has never been demonstrated in butterflies. Here we describe a new type of ultrasound-sensitive ear that we have discovered in an unusual group of nocturnally active, neotropical butterflies, the Hedyloidea. Hedylids have ears on their wings and respond to ultrasound by making flight manoeuvres to avoid bats. On the basis of phylogenetic and comparative anatomical evidence, we propose that hearing in Lepidoptera and day flight in butterflies both result from an intense selection pressure imposed by echolocating bats more than 50 million years ago.
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