Flapping ears

For nocturnal insects, predation by bats can turn a moonlight flight into a nightmare. Moths have a way of avoiding bats, however, in the form of ultrasonic hearing. Because they have 'tympanal' ears, they can hear the echolocation calls of incoming bats and take evasive action. Butterflies, which are mainly diurnal, have not been known to have ultrasonic hearing. But these images from an unusual nocturnal butterfly, the hedylid, show that these butterflies, at least, have tympanal ears. The lower image shows the ventral surface of the forewing base and the cavity in which the ear is located. A fringe of scales over the cavity covers the tympanal membrane. Because the ears are located on the leading edge of the forewing they move with the wing during flight, perhaps aiding in sound detection. Sound-induced vibrations of the tympanal membrane are detected by three sensory organs, called chordotonal organs (the thin horizontal structures in the top image). These attach to the inner surface of the tympanal membrane; attachments for two of the three chordotonal organs can be seen to the right. To the left, the chordotonal organs are attached to a base structure within the tympanic chamber. The chordotonal organs themselves are composed of collections of individual sensory units called scolopidia. As the Hedyloidea are possible 'living ancestors' of modern-day butterflies and are considered their closest relatives, the identification of their ability to hear ultrasound may have implications for the evolutionary origins of butterflies.