Abstract

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.