Unlike humans, grasshoppers do not have ears on the side of their head. Yesterday, I posted about “ears” on their front legs of Katydids. Grasshoppers have a different location for their ears, the sides of the abdomen.
In the giant lubber grasshopper, the abdominal segment next to the thorax (behind the third pair of legs) contains the grasshopper sound detection organ. Like the ears of people and the “ears” of katydids, the grasshopper sound detector is a thin membrane called a tympanum. (People often call it the “ear drum”). The tympanum, like the surrounding cuticle is a cellular secretion, but its properties are different. The membrane is tough, but not as hard or thick as the surrounding cuticle. Although difficult to puncture, the membrane is less strong than cuticle. In adults, the tympanum is covered and protected by the wings.
The tympanum has characteristics that are similar to a drum. Beating on a drum with a mallet will vibrate the drum head (membrane) to create a sound. Drum heads can also respond to sounds. When sound waves of the correct frequency and amplitude hit the drum head, the drum will vibrate. Good drummers know this. If you watch the drummers in an orchestra, you will see that in quiet passages where the drums are silent, the drummers will place a finger or hand on the drum head to dampen the vibration and keep the drum quiet.
The grasshopper tympanum is adapted to vibrate in response to signals that are important to the grasshopper. Male grasshoppers use sounds to call for mates and to claim territory. Females can hear the sound that males make and judge the relative size of the male from the pitch of the call (large males have make deeper sounds). Other males can hear the sounds and judge the size of a potential rival. Males use this information to avoid fights with larger male grasshoppers or to chase smaller rivals from their territory.