Auditory processing in the black-sided meadow katydid Conocephalus nigropleurum (Orthoptera: Tettigoniidae)

1. The auditory system of the tettigoniid,Conocephalus nigropleurum was examined using whole auditory nerve responses (averaged summed action potentials (SAPs)) and intracellularly recorded single cells (receptor and interneurone) to pure tone stimuli and con- and heterospecific (C. brevipennis) songs.

2.The morphology of the auditory tracheal system and crista acustica is documented forC. nigropleurum. A broadly open thoracic spiracle (stigma) leads, via a horn, camera and foreleg trachea to a crista acustica of 28 receptor cells.

3. C. nigropleurum's auditory sensitivity is broadly tuned to 15–35 kHz which is below the peak frequency band of its calling song (30–36 kHz). The ear possesses a dynamic range of at least 30 dB and pulsed sounds with short (<0.5 ms) onset times or long interpulse periods (>200 ms) elicit maximal responses. We suggest this is caused by increased auditory receptor firing synchrony to these types of sounds and may facilitate the location of such sounds. There is, however, discrepancy between the pulse periods that elicit maximal SAP amplitudes and those found within the normal calling song, a phenomenon which may be related to the chorusing behaviour of this species.

4.Polar directional plots indicate thatC. nigropleurum is most sensitive to sounds originating posterior to the opening of the prothoracic auditory stigma; this may be caused by the configuration of the prothoracic pronotum. We found no pronounced sensitivity at points facing the tympanal slits.

5.Intracellular recordings reveal prothoracic receptors with widely decussate and narrowly branched termini. No receptor tested was able to track the intra-phonatome pulses (tooth strike sounds) of either species and we conclude that the phonatome is the finest resoluble component of the song.

6. An acoustically-activated interneurone (101) was recorded that resembles both the gryllid int-2 (Omega) cell and that of another tettigoniid. We suggest that tonic interneuronal firing caused by the multiple singer chorused song of either species activates the initial stages of phonotaxis in receptiveC. nigropleurum females but individual species recognition may depend at least partially on the discrimination of phonatome rates.