On the tympanic membrane impedance of the katydid Copiphora gorgonensis: (Insecta: Orthoptera: Tettigoniidae)

Katydids (bush-crickets) are endowed with tympanal ears located in their forelegs' tibiae. The tympana are backed by an air-filled tube, the acoustic trachea, which transfers the sound stimulus from a spiracular opening on the thorax to the internal side of the tympanic membranes (TM). In katydids the sound stimulus reaches both the external and internal side of the membranes, and the tympanal vibrations are then transferred to the hearing organ crista acustica (CA) that contains the fluid-immersed mechanoreceptors. Hence the tympana are principally involved in transmitting and converting airborne sound into fluid vibrations that stimulate the auditory sensilla. Consequently, what is the transmission power to the CA? Are the TM tuned to a certain frequency? To investigate this, the surface normal acoustic impedance of the TM is calculated using finite-element analysis in the katydid Copiphora gorgonensis. From this, the reflectance and transmittance are obtained at the TM. Based on the impedance results obtained from the pressure recordings at TM and the velocity field calculations in the AT, in the frequency range 5–40 kHz, it is concluded that the tympana have considerably higher transmission around 23 kHz, corresponding to the dominant frequency of the male pure-tone calling song in this species.