The human ear is a familiar sound detection organ. The outer, visible part of our ear acts like a giant funnel that directs sound waves into the ear canal. Our ear canal is a short tunnel that leads to our eardrum, a fragile membrane (tympanum) that vibrates when stimulated by sound waves of appropriate frequency and amplitude. Our ears are fine tuned to detect sounds that other humans make for communication and sounds made by potential enemies and dangers.
Our ears are not our only means of detecting sound. We also can detect sounds (vibrations) of selected frequency and volume with nerves that are sensitive to vibrations of our body. Most Americans have encountered this phenomena at stoplights when a car with very loud bass pulls alongside. The bass tones vibrate the air, vibrate our car and seat, and can be detected as vibration by our body. Although we can “hear” the bass tones with our ears, we don’t need ears to detect the vibrations.
Most insects are capable of detecting vibrations. In addition, groups of insects that use sounds for communication have specialized sound detection organs. Most of us are familiar with the sound of katydids and crickets as they sing in the night air. The sound is the mating call of males who are serenading the females in hopes of attracting a mate. The singing as well as the mate finding takes place under cover of darkness.
Female katydids do not have flashlights and ambient light is often too dim to detect a potential mate. The katydids must rely on sound detection to find a mate. Katydids have well developed sound detection organs on their legs. The sound detector is a shallow pit in the cuticle with a tympanum (katydid eardrum) that will vibrate at the frequencies used by male katydids. This “tunes in” the mating calls of the males and “tunes out” background noise. Why are the sound detectors located on the legs?
If properly placed, sound detectors can determine where a sound originates. For systems with two sound detectors, the greater the distance between the detectors, the more accurately they can determine the direction of the sound. In their location on the front legs, the sound detectors of a katydid are located as far apart as practical. The front legs are not in the “sound shadow” of the body and have a clear path to the origin of the sound.
The katydid “ears” are directionally sensitive. Sounds originating from in front of the katydid more directly impact the tympanum and thus are louder than sounds originating from behind (and partly blocked by the back of the leg). If a male is calling from behind the female, turning around will make the sound louder.
The sound detectors do not face straight forward to the same location but forward at a slight angle to each other. In this configuration, the volume detected by the left leg is equal to the volume detected by the right leg when the female is directly facing the origin of the sound. By equalizing the volume in the left and right leg, the katydid knows it is headed in the right direction even in the dark of the darkest night.