Comparative micromechanics of bushcricket ears with and without a specialized auditory fovea region in the crista acustica

Abstract

In some insects and vertebrate species, the specific enlargement of sensory cell epithelium facilitates the perception of particular behaviourally relevant signals. The insect auditory fovea in the ear of the bushcricket Ancylecha fenestrata (Tettigoniidae: Phaneropterinae) is an example of such an expansion of sensory epithelium. Bushcricket ears developed in convergent evolution anatomical and functional similarities to mammal ears, such as travelling waves and auditory foveae, to process information by sound. As in vertebrate ears, sound induces a motion of this insect hearing organ (crista acustica), which can be characterized by its amplitude and phase response. However, detailed micromechanics in this bushcricket ear with an auditory fovea are yet unknown. Here, we fill this gap in knowledge for bushcricket, by analysing and comparing the ear micromechanics in Ancylecha fenestrata and a bushcricket species without auditory fovea ( Mecopoda elongata , Tettigoniidae: Mecopodinae) using laser-Doppler vibrometry. We found that the increased size of the crista acustica, expanded by a foveal region in A. fenestrata , leads to higher mechanical amplitudes and longer phase delays in A. fenestrata male ears. Furthermore, area under curve analyses of the organ oscillations reveal that more sensory units are activated by the same stimuli in the males of the auditory fovea-possessing species A. fenestrata . The measured increase of phase delay in the region of the auditory fovea supports the conclusion that tilting of the transduction site is important for the effective opening of the involved transduction channels. Our detailed analysis of sound-induced micromechanics in this bushcricket ear demonstrates that an increase of sensory epithelium with foveal characteristics can enhance signal detection and may also improve the neuronal encoding.

Reconstruction of sound driven, actively amplified and spontaneous motions within the tree cricket auditory organ

“…It is thus possible that the motion observed in the katydid organ (Montealegre-Z. et al, 2012;Scherberich et al, 2020) may also be a structural rather than a fluid wave or may contain both elements, which would be more similar to current models of the mammalian cochlea (Bowling and Meaud, 2018;Meaud and Grosh, 2012). We propose that OCT based depth resolved vibrometry would be a more sensitive technique than surface-level vibrometry to test the similarities and differences between these organs.…”

“…We propose that the OCT measurement paradigm we present here would provide a more sensitive method to test whether both processes may be present simultaneously in both or either group. As now demonstrated in Tettigoniids, OCT is invaluable to measuring internal mechanics in detail (Scherberich et al, 2020(Scherberich et al, , 2016Vavakou et al, 2021) and will be crucial in teasing apart the different contributing mechanisms that might determine the center frequency and sharpness of auditory tuning.…”

“…The stopping location of each frequency is tonotopically arranged, thus each frequency excites a distinct set of mechanosensory neurons (Montealegre-Z. et al, 2012;Scherberich et al, 2020).…”

“…The mechanics of the Tettigoniid auditory organ have been proposed to be quite distinct and have some features that strongly resemble the auditory mechanics of the mammalian cochlea (Montealegre-Z. et al, 2012;Scherberich et al, 2020). In this system, it has been suggested that a part of the tympanal membrane acts as a lever, pushing on the fluid in the auditory vesicle.…”

Analysis of fixation materials in micro-CT: It doesn’t always have to be styrofoam

“…Micro-computed tomography has been used as an imaging technique for many years [1] and has become a standard procedure for evaluating small samples in many disciplines. These include material sciences [2][3][4][5][6][7][8], mineral and soil science [9][10][11][12], or life sciences [13][14][15] like the analysis of small structures of various animal models [16][17][18][19][20][21]. Furthermore there are many more clinical use cases such as forensics [22] and bone analysis [23][24][25][26][27][28].…”