The biophysics and biochemistry of a cochlea-like organ in the ear of Neotropical bush-crickets (Insecta: Tettigonidae)

There has been an increasing interest in the study of complex auditory processes in the mammalian cochlea (e.g. frequency resolution, frequency discrimination and active amplification). These processes depend on the propagation of frequency information in the form of travelling waves (of the type exemplified in a tsunami) along the tonotopically arranged auditory sensilla. The physiological and biophysical bases of traveling waves in the mammalian cochlea remain elusive, yet vital to understanding tonotopy (the mapping of sound frequency across space) and active amplification. In vertebrates, both location and osseous protective material make the inner ear difficult to access without altering its integrity. While conventional methods for hearing research in vertebrates have improved notably in recent years, these still 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 small surgical openings or other isolated places. Remarkably, complex auditory processes are not unique to vertebrates, and similar mechanisms for sound filtering, amplification, and frequency analysis have also been found in the ears of insects. Hearing organs in insects are unusually small, highly sensitive, and easily accessible by means of non-destructive methods. Among insects, bushcrickets (Insecta: Orthoptera) have a unique hearing system which consists of minute tympanal ears located in the forelegs, and inner ears with tonotopically organised auditory sensilla within a fluid-filled cavity. Unlike in vertebrates, the bush-cricket inner ear is not coiled, but stretched. Critically, the assessment of auditory processes in this small-scale ear is proposed to be possible in a non- vi invasive manner. The purpose of this thesis was to further the knowledge of acoustic perception in bush-crickets by providing new data on the travelling wave phenomenon, the suitability of bush-crickets for non-invasive experimentation, and the elemental composition of the liquid contained in the bush-cricket inner ear. It was demonstrated that transparency is the cuticle property that allows the observation and measurement of travelling waves and tonotopy in bush-crickets through the use of light measurement techniques, specifically laser Doppler vibrometry. This approach provides a non-invasive alternative for measuring the natural motion of the sensillia-bearing surface embedded in the intact inner ear’s fluid. Subsequently, this experimental technique was used to generate novel data on inner ear mechanics from a number of bush-cricket species. Finally, in the form of a chemical analysis, I established that the inner ear’s liquid differs from the hemolymph based on the variation of their ion concentration values. From a biomechanical perspective, the presence of a liquid-filled cavity along with a species-specific ion concentration, likely contributes to an optimal functioning of the hearing organ just as it occurs in vertebrates. These results highlight the importance of considering analogous models of vertebrate hearing systems for advanced studies 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.