Hearing in a primitive ensiferan: the auditory system of Cyphoderris monstrosa (Orthoptera: Haglidae)

Abstract

1. Insects of the family Haglidae are relicts of primitive Ensifera. Members of this group display acoustic behaviour in the typical ensiferan pattern, with males producing stereotyped acoustic signals by tegminal stridulation. They are closely related to the major groups of acoustic Ensifera, but their auditory system has not been previously studied. I present here a description of the morphology, and physiology of the peripheral auditory system of Cyphoderris monstrosa. 2. The tympana of C. monstrosa are similar to those of Tettigoniidae, but less well-differentiated from normal leg cuticle. The acoustic trachea is well-developed, but does not form a single, continuous tube in the leg proximal to the tympana. The organization of the auditory organ is similar to the Tettigoniidae, with the tibial organ comprising the subgenual organ, intermediate organ, and crista acustica. Projections of the tibial organ sensory cells to the prothoracic ganglion are also typical of acoustic Orthoptera. The meso- and metathoracic legs possess tibial organs similar to that of the prothoracic leg, although lacking tympana and well-developed acoustic tracheae. Central projections of the mesothoracic tibial organ are similar to those of the prothoracic. 3. Auditory whole nerve responses are not tuned to the frequency of the species calling song. Auditory best frequency (BF) is 2 kHz; calling song frequency is 12 kHz. Threshold intensity levels at BF are typical for insect hearing organs (35–45 dB SPL). The response of the auditory organ is not strongly directional at the calling song frequency. Occlusion of the acoustic trachea has no effect on the sensitivity, tuning, or directionality of the auditory system. Occlusion of either of the tympana alone also has no effect on auditory responses, but occlusion of both tympana leads to a marked decrease in sensitivity. Tuning is not affected. The tibial organ of the mesothoracic leg is sensitive to airborne sound at low frequencies (BF = 1 kHz) with thresholds comparable to the ear. 4. The auditory system of C. monstrosa is a pressure receiver, adapted to low frequency sensitivity. This low frequency tuning, in contrast to the calling song frequency, suggests an auditory system adapted to another function in addition to intraspecific communication.