Insects are terrestrial, but their crustacean ancestors were aquatic. Organs for detecting underwater sounds are poorly suited to detecting airborne sounds. Early insects were deaf and many modern insects do not respond to airborne sounds. Over 350000 species of beetles are deaf. Exceptions include a few genera of Scarabs and Tiger Beetles.
Detection of airborne sounds can be useful for avoiding predation by bats. Bats hunt by echolocation, emitting an ultrasound that bounces off insects and other objects. Bats use the reflected sound to locate prey. Flying insects that detect bat ultrasound allows insects can avoid predation.
Forrest and colleagues* noted that a night flying scarab beetle, Euetheola humilis, the sugarcane beetle, took evasive action when bat ultrasounds were broadcast over a speaker. Beetles halted their flight and did not resume flying in the presence of the bat ultrasound. Forrest & colleagues investigated further and located tympanum type hearing organs in the cervical membrane between head and thorax. They investigated related beetle species and found similar organs in three genera of Scarabs.
Hearing evolved in these night flying Scarabs as a defense against bats. Flightless beetle that are not at risk from bat predation can enjoy the silence.
T. G. FORREST; M. P. READ; H. E. FARRIS & R. R. HOY. 1997. A TYMPANAL HEARING ORGAN IN SCARAB BEETLES. The Journal of Experimental Biology 200, 601–606.