The biophysics and biochemistry of a cochlea-like organ in the ear of Neotropical bush-crickets (Insecta: Tettigonidae)
Creators: name: Sarria, Fabio
Publisher: name: University of Lincoln
Year: 2023

description: There has been an increasing interest in the study of complex auditoryprocesses in the mammalian cochlea (e.g. frequency resolution, frequencydiscrimination and active amplification). These processes depend on thepropagation of frequency information in the form of travelling waves (of thetype exemplified in a tsunami) along the tonotopically arranged auditorysensilla. The physiological and biophysical bases of traveling waves in themammalian cochlea remain elusive, yet vital to understanding tonotopy (themapping of sound frequency across space) and active amplification. Invertebrates, both location and osseous protective material make the inner eardifficult to access without altering its integrity. While conventional methods forhearing research in vertebrates have improved notably in recent years, thesestill require surgical procedures to gain physical access to the inner ear,compromising the natural conditions of the hearing system. Indeed,measurement of auditory activity in-vivo has only been done through smallsurgical openings or other isolated places. Remarkably, complex auditoryprocesses are not unique to vertebrates, and similar mechanisms for soundfiltering, amplification, and frequency analysis have also been found in the earsof insects. Hearing organs in insects are unusually small, highly sensitive, andeasily accessible by means of non-destructive methods. Among insects, bushcrickets (Insecta: Orthoptera) have a unique hearing system which consists ofminute tympanal ears located in the forelegs, and inner ears with tonotopicallyorganised auditory sensilla within a fluid-filled cavity. Unlike in vertebrates, thebush-cricket inner ear is not coiled, but stretched. Critically, the assessment ofauditory processes in this small-scale ear is proposed to be possible in a non-viinvasive manner. The purpose of this thesis was to further the knowledge ofacoustic perception in bush-crickets by providing new data on the travellingwave phenomenon, the suitability of bush-crickets for non-invasiveexperimentation, and the elemental composition of the liquid contained in thebush-cricket inner ear. It was demonstrated that transparency is the cuticleproperty that allows the observation and measurement of travelling waves andtonotopy in bush-crickets through the use of light measurement techniques,specifically laser Doppler vibrometry. This approach provides a non-invasivealternative for measuring the natural motion of the sensillia-bearing surfaceembedded in the intact inner ear’s fluid. Subsequently, this experimentaltechnique was used to generate novel data on inner ear mechanics from anumber of bush-cricket species. Finally, in the form of a chemical analysis, Iestablished that the inner ear’s liquid differs from the hemolymph based on thevariation of their ion concentration values. From a biomechanical perspective,the presence of a liquid-filled cavity along with a species-specific ionconcentration, likely contributes to an optimal functioning of the hearing organjust as it occurs in vertebrates. These results highlight the importance ofconsidering analogous models of vertebrate hearing systems for advancedstudies of auditory function. Such models can be used to effectively observe,collect, and measure auditory data otherwise impossible to attain noninvasively in vertebrates, and specifically mammalian species.