Hawkmoths fly at night and are tasty prey for bats. Some hawkmoths have evolved the ability to detect the ultrasonic sounds emitted by bat sonar. The Death Head Hawkmoth and some other moths in the Choerocampina, use a sound detection organ that has evolved from a modified labial palp and labral pilifer (a hair covered remnant of the vestigial mandibles). The labral pilifer contains the sensory nerves that send signals to the brain. The palp is modified to vibrate in response to sound. The palp has a sound sensitive segment that is enlarged and contains an air sac. The cuticle covering the air sac is thin and devoid of scales. The thin membrane backed by an air sac forms a tympanum that vibrates in response to select sound frequencies.
The image on the left shows the right palp in its natural position where it contacts the pilifer. The left palp was moved to reveal the pilifer. When the sound organ on the palp vibrates, it contacts the pilifer. The movement causes the pilifer to send a signal to the brain. This unusual hearing organ is only found in a few species of hawkmoth.
Many species of insects can eavesdrop on bats and do so by a variety of sound detectors that have evolved independently. The number of bat detector types is testament to the toll that bats take on flying insects and the advantages to insects of an early warning system.
Gopfert MC,Wasserthal LT. 1999. Hearing with the mouthparts: behavioral responses and the structural basis of ultrasound perception in Aherontiine hawkmoths. J. Exp. Biol. 202:909–18