A numerical approach to investigating the mechanisms behind tonotopy in the bush-cricket inner-ear

Abstract

Bush-crickets (or katydids) have sophisticated and ultrasonic ears located in the tibia of their forelegs, with a working mechanism analogous to the mammalian auditory system. Their inner-ears are endowed with an easily accessible hearing organ, the crista acustica (CA), possessing a spatial organisation that allows for different frequencies to be processed at specific graded locations within the structure. Similar to the basilar membrane in the mammalian ear, the CA contains mechanosensory receptors which are activated through the frequency dependent displacement of the CA. While this tonotopical arrangement is generally attributed to the gradual stiffness and mass changes along the hearing organ, the mechanisms behind it have not been analysed in detail. In this study, we take a numerical approach to investigate this mechanism in the Copiphora gorgonensis ear. In addition, we propose and test the effect of the different vibration transmission mechanisms on the displacement of the CA. The investigation was carried out by conducting finite-element analysis on a three-dimensional, idealised geometry of the C. gorgonensis inner-ear, which was based on precise measurements. The numerical results suggested that (i) even the mildest assumptions about stiffness and mass gradients allow for tonotopy to emerge, and (ii) the loading area and location for the transmission of the acoustic vibrations play a major role in the formation of tonotopy.

The chemistry of an insect ear: ionic composition of a liquid-filled ear and haemolymphs of Neotropical katydids

“…Although poorly understood, katydid ears with reported AV might be endowed with a softer enclosure than the vertebrate cochlea. This enclosure is provided by the dorsal cuticle and surrounding structure such as the tympanal membranes, acoustic trachea and the colloidal material at each end of the inner ear cavity [15,27]. royalsocietypublishing.org/journal/rsif J. R. Soc.…”

“…Based on anatomical studies of the hearing system of C. gorgonensis, Montealegre-Z et al [15] observed that the haemolymph channel in this species was not continuous as had been suggested before, but instead, they found a fluid trapped in a cavity bounded by the presence of a colloidal material at each end of the auditory organ. This cavity is the AV, and it is considered to be an important element of the auditory system as it provides a medium for the propagation of mechanical travelling waves and influences the dynamics of the tympanal membranes [19,27]. A fluid-filled cavity appears to be a required condition for the function of auditory organs in some Neotropical katydids.…”

Beyond the exponential horn: a bush-cricket with ear canals which function as coupled resonators

“…The constructed STL files for the trachea geometries used in the numerical models (for six specimens) and the set-up of the three different numerical models (developed in COMSOL Multiphysics, v. 5.6) are available from the Dryad Digital Repository: [51].…”