Spectral selectivity during phonotaxis: a comparative study in Neoconocephalus (Orthoptera: Tettigoniidae)

The calls of male Neoconocephalus have most energy concentrated in a relatively narrow low-frequency band. In N. robustus this low-frequency band is centered around 7 kHz, whereas calls of N. nebrascensis and N. bivocatus have center frequencies close to 10 kHz. The importance of the position of the low-frequency band for female phonotaxis in these three species was determined using a walking compensator. Female N. robustus showed significant phonotaxis towards call frequencies from 5 to 10 kHz, and spectral selectivity towards higher frequencies did not change with stimulus amplitude. Significant responses in N. nebrascensis and N. bivocatus occurred at significantly higher frequency ranges than in N. robustus. In these species, spectral selectivity changed with stimulus amplitude; at 68 dB sound pressure level (SPL), upper cut-off frequency was significantly lower than at 80 dB SPL in both species. Adding a higher harmonic to the conspecific carrier frequency had a strong inhibitory effect on phonotaxis in N. robustus: at higher relative amplitudes of the harmonic, phonotaxis was completely suppressed. Adding a higher harmonic to the conspecific carrier frequency had a much weaker but significant inhibitory effect in N. nebrascensis and little, if any, effect in N. bivocatus. The processing of song spectrum in the sensory system is discussed with regard to the differences in spectral selectivity among the three species. The sharp spectral selectivity of N. robustus is interpreted as an adaptation for species isolation.