What determines the tuning of hearing organs and the frequency of calls? A comparative study in the katydid genus Neoconocephalus (Orthoptera, Tettigoniidae).

The calls of five syntopic species of Neoconocephalus varied significantly in their spectral composition. The center-frequency of the narrow-band low-frequency component varied from 7 kHz to 15 kHz among the five species. Hearing thresholds, as determined from whole nerve recordings, did not vary accordingly among the five species but were lowest in the range from 16 kHz to 18 kHz in all five species. Iso-intensity response functions were flat for stimulus intensities up to 27 dB above threshold, indicating an even distribution of the best frequencies of individual receptor cells. At higher stimulus intensities, the intensity/response functions were steeper at frequencies above 35 kHz than at lower frequencies. This suggests the presence of a second receptor cell population for such high frequencies, with 25-30 dB higher thresholds. This receptor cell population is interpreted as an adaptation for bat avoidance. The transmission properties of the Neoconocephalus habitat (grassland) had low-pass characteristics for pure tones. Frequencies below 10 kHz passed almost unaffected, while attenuation in excess of spherical attenuation increased at higher frequencies. Considering these transmission properties and the tuning of female hearing sensitivity, call frequencies of approximately 9-10 kHz should be most effective as communication signals in this group of insects. It is discussed that the frequency of male calls is strongly influenced by bat predation and by the transmission properties of the habitat but is not strongly influenced by the tuning of the female hearing system.