Auditory mechanics in a bush-cricket: direct evidence of dual sound inputs in the pressure difference receiver

Abstract

The ear of the bush-cricket, Copiphora gorgonensis, consists of a system of paired eardrums (tympana) on each foreleg. In these insects, the ear is backed by an air-filled tube, the acoustic trachea (AT), which transfers sound from the prothoracic acoustic spiracle to the internal side of the eardrums. Both surfaces of the eardrums of this auditory system are exposed to sound, making it a directionally sensitive pressure difference receiver. A key feature of the AT is its capacity to reduce the velocity of sound propagation and alter the acoustic driving forces at the tympanum. The mechanism responsible for reduction in sound velocity in the AT remains elusive, yet it is deemed to depend on adiabatic or isothermal conditions. To investigate the biophysics of such multiple input ears, we used micro-scanning laser Doppler vibrometry and micro-computed X-ray tomography. We measured the velocity of sound propagation in the AT, the transmission gains across auditory frequencies and the time-resolved mechanical dynamics of the tympanal membranes in C. gorgonensis Tracheal sound transmission generates a gain of approximately 15 dB SPL, and a propagation velocity of ca 255 m s, an approximately 25% reduction from free field propagation. Modelling tracheal acoustic behaviour that accounts for thermal and viscous effects, we conclude that reduction in sound velocity within the AT can be explained, among others, by heat exchange between the sound wave and the tracheal walls.

Phylogenomic analysis sheds light on the evolutionary pathways towards acoustic communication in Orthoptera

“…Extant ensiferan ears usually have two auditory inputs, with sound arriving at the external surface of the tibial tympana, and also internally via the acoustic trachea, which open on the acoustic spiracles at the side of the pronotum 44 , 70 – 72 . These ears are pressure difference receivers 73 , 74 , as the sound travelling internally on the trachea travels slower and a longer distance than that reaching the external surface of the tympanum from the outside at the normal speed of sound propagation in air. This causes differences in gain between sound arriving externally and internally 72 , 74 , 75 .…”

Two Mysterious Tiny Katydids from the Ecuadorian Andes (Orthoptera: Tettigoniidae: Hexacentrinae)

“…In Nubimystrix the tympana are totally unconcealed. For the ears to work as pressure difference receivers, wide auditory spiracles would be necessary, with the narrowing auditory trachea connected to the inner surface of the tympana acting as sound amplifying exponential horn (Jonsson et al 2016). However, with moderately high ultrasound, a small auditory spiracle might be sufficient (Fernando Montealegre-Z., pers.…”

Frequency tuning and directional sensitivity of tympanal vibrations in the field cricketGryllus bimaculatus

“…In this view, amplitudes and phase shifts for the internal transmissions are optimized to provide peak sensitivity at the calling song frequency, thus boosting directional sensitivity [23,28]. Similar modulations of amplitude and phase for sounds transmitted in the acoustic trachea have been shown for bush crickets [29]. Transmission from the contralateral spiracles involves an additional phase shift due to the medial septum [30,31], which varies steeply with sound frequency [9,32].…”