Most of the 350000 species of beetles are deaf. Only two groups of beetles are known to detect airborne sounds: a few night flying scarab beetles and some tiger beetles*. Both groups of beetles display bat evasion behaviors triggered by bat ultrasounds.
Detection of bat sounds evolved independently in both groups. Existing membranes have been altered to form a tympanum capable of sound discrimination. The acoustic detectors in scarabs are located in the cervical membrane between head and thorax. Acoustic detectors in tiger beetles are located in the first abdominal segment.
Tiger beetle elytra cover the hearing detectors and block the sound when running along the ground. In flight, the elytra are raised exposing the detectors to bat sonar. When bat sounds are broadcast, the beetles alter their flight mechanics, a bat avoidance response. The beetles also generate a “clicking” sound which may have defensive properties of confusing the bat or disrupting its sonar.
*Yager, D D ; Spangler, H G. Behavioral response to ultrasound by the tiger beetle Cicindela marutha combines aerodynamic changes and sound production. The Journal of experimental biology, February 1997, Vol.200(Pt 3), pp.649-59