Insects have evolved hearing numerous times in at least 7 orders and sometimes multiple times within an order and it is useful to compare and contrast similarities among the instances. In each of these cases there is an ear drum, the tympanal membrane, backed by an air sac. In most cases of insect hearing the receptor neurons are directly attached to the membrane, deflecting with sound.
I am currently studying one of the simplest ears, that of the noctuid moth. The ears on these moths are located behind the wing on the thorax (A). They only have two auditory neurons per ear (B & C arrows), yet transmit enough information to help the moth determine if it should turn away from a distant bat or fly erratically from a rapidly approaching bat.
Locusts have their ears on their first segment of their abdomen. They have about 70 neurons per ear and attach to the tympanal membrane in three main locations. The high frequencies are primarily received at the pyrifom vesicle, seen as a darker pigmented dot in the center. Locusts receive sound as a traveling wave. At low frequencies the membrane has a large wave that passes over the membrane. At higher frequencies, the traveling wave has a much shorter period and peaks at the pyrifom vesicle.