Most hearing insects have a pair of tympanal organs. Think of a tympani, the large drum used in the percussion section of an orchestra. Like the tympani, the tympanal organ consists of a membrane stretched on a frame, over an air-filled cavity. When the percussionist hammers on the membrane of the tympani, it vibrates and produces a sound. The insect's tympanal organ vibrates as it catches sound waves in the air. The insect also has a special receptor called the chordotonal organ, which senses this vibration of the tympanal organ, and translates the sound into a nerve impulse. Insects that use tympanal organs to hear include grasshoppers and crickets, cicadas, and some butterflies and moths.
For many insects, a receptor on the antennae called the Johnston's organ collects auditory information. Sensory cells on the pedicel, which is the second segment from the base of the antennae, detect vibration of the segment(s) above. Mosquitoes and fruit flies hear using the Johnston's organ.
The larvae of Lepidoptera and Orthoptera use small hairs, called setae, to gather the vibrations of sound. Caterpillars often respond to such sounds by exhibiting defensive behaviors. Some will stop moving completely, while others may contract their muscles and rear up in a fighting posture.
A structure in the mouths of certain hawkmoths enables them to hear ultrasonic sounds, such as those produced by echolocating bats. The labral pilifer, a tiny hair-like organ, is believed to sense vibrations at specific frequencies. Scientists note a distinctive movement of the insect's tongue when they subject the captive hawkmoths to those sounds. In flight, the hawkmoths can avoid a pursuing bat by listening for these frequencies.