A complex mechanism of call recognition in the katydid Neoconocephalus affinis (Orthoptera: Tettigoniidae)

Abstract

Acoustic pattern recognition is important for bringing together males and females in many insect species. We used phonotaxis experiments on a walking compensator to study call recognition in the katydid Neoconocephalus affinis, a species with a double-pulsed call and an atypically slow pulse rate for the genus. Call recognition in this species is unusual because females require the presence of two alternating pulse amplitudes in the signal. A Fourier analysis of the stimulus-envelopes revealed that females respond only when both the first and second harmonics of the AM spectrum are of similar amplitude. The second harmonic is generated by the amplitude difference between the two pulses making up a pulse-pair. Females respond to double pulses that have been merged into a single pulse only if this amplitude modulation is preserved. Further experiments suggest that females use a resonance mechanism to recognize the pulse rate of the call, supporting a neural model of rate recognition in which periodic oscillations in membrane potential are used to filter the pulse rate of the signal. Our results illustrate how a reduction in pulse rate extends the opportunities for females to evaluate fine-scale temporal properties of calls, and provide further evidence for the importance of oscillatory membrane properties in temporal processing. The results are discussed with regard to evolutionary changes in call recognition mechanisms within the genus.