Nanomechanics of Insect Tympanal Sound Receivers

This project has investigated the mechanical characteristics of tympanal ears in insects, specifically the locust and moth. The main part of this project has been the investigation of frequency analysis in the locust tympanal membrane. Hearing animals have the capacity to analyse the frequency composition of sound. In mammals, this frequency analysis relies on the mechanical response of the basilar membrane in the cochlear duct. These vibrations take the form of a slow vibrational wave, known as von Békésy's travelling wave, propagating along the basilar membrane from base to apex. The wave displays amplitude maxima at frequency-specific locations along the membrane, providing a spatial map of the frequency of sound. Insect auditory systems may not be as structurally sophisticated as those of mammals, yet some are known to perform sound frequency analysis. In the desert locust, we have shown that this analysis arises from the mechanical properties of the tympanal membrane. The spatial decomposition of incident sound involves a tympanal travelling wave. This funnels mechanical energy to specific tympanal locations, where distinct groups of mechanoreceptor neurones project. The locust?s ear thus combines in one structure the functions of sound reception and frequency decomposition.

Nanomechanics of Active Sensors in Insects

This new research project will investigate the mechanical and neural processes at work in the auditory organs of insects. Nanoscale and sub-nanoscale auditory sensitivity will be investigated with respect to both auditory mechanics and neuronal motility. A range of insects, such as mosquitoes for example, will be used, as despite their small size and apparent relative simplicity, they display many of the functional attributes of the auditory organs of mammals, and in particular active auditory mechanics. This latter process is currently one of the most debated topics in auditory science. In essence, the research programme will examine every step in the chain of hearing, conducting a thorough system’s analysis of the insect’s auditory system. This will start with a mechanical study of how the hearing organ translates changes in air particle velocity into the motion of mechanoreceptor cells, culminating with an explanation of the transduction of energy through the mechanoreceptor structure. A further step will entail combining mechanical stimuli, either to the antenna or to the mechanoreceptor, with electrophysiological measurements of the neural impulses generated by the mechanoreceptor neurone. Thus a direct causal connection between mechanical stimulation and neural activity will be made.

Mechanical Characterisation of Mosquito Antenna

The antennal hearing system of the male mosquito is arguably the most sensitive among insects capable of detecting sinusoidal vibrations down to the nanometre scale. This hearing ability is used to detect female mosquitoes by the sound of their wing-beat so it is vital for them to preserve the antenna's function and sensitivity. However, the nanoscale sensitive antennae are borne on the head of the animal, unprotected from possible physical shocks and deflections. Too large a deflection or shock could damage the flagellum by bending it too far or breaking it, therefore making it unable to receive sound. It has been suggested that mosquitoes have evolved a mechanism that will protect the sensitive mechanoreceptor organ (called the Johnston's organ) from external impacts. If this is proved to be the case, the antenna may have some attractive attributes that can be applied to AFM nanotechnology. On one hand, it can be said that current silicon AFM cantilevers are 'man-made antennae' in that they are also mechanical force or displacement detectors with nanoscale sensitivity. On the other hand, the brittleness of silicon means that AFM cantilevers need to be handled with a great deal of care as they are easily damaged or broken. If the robustness of the mosquito antenna can be applied in the development of AFM cantilevers it could increase the typical lifetime of silicon AFM cantilevers.

Mosquito Audition: A Nonlinear Nanomechanical Sensor

The male mosquito detects the sound produced by a female in order to locate a mate. It achieves this through the use of a plumose antenna that oscillates in the presence of an inherently weak sound field. Despite noise and stochastic firing of neurons, the mosquito can detect antennal displacements as little as 7nm. Although structurally adapted for sound detection, the mosquito further improves its sensitivity through active control. Amplification, quality factor control, and acute sensitivity are hallmarks of active hearing, and the mosquito is no exception. This project aims to understand the nonlinear dynamics involved in active hearing in the mosquito, with emphasis on collective motion of mechanosensory neurons that both sense and actuate motion of the antenna.

Experiments in Fly Audition

The antennae in several insect orders have been found to fulfil an important role in sensing air particle displacement. Early work showed that Calliphora could infer their flight speed from the airflow deflecting their antennae during flight. It has also been proposed that the antennae can detect the air currents generated by the fly's own wing strokes. In addition, there is evidence that the antenna responds to sound. Female Drosophila melanogaster use their antennae to detect male courtship songs. Similarly the antennae of male mosquitoes are tuned to the frequency of the female acoustic wing-beat for mate location. It has been suggested that antennal sensitivity to sound may be widespread amongst Diptera. If true, we may ask whether sound is used in other roles besides courtship and mating behaviour. For example, H.C. Bennet-Clark speculates whether, “using their antennae, flies are sensing the changes in the self-generated flight tone that are brought about by the echoes of looming objects”. Bennet-Clark thus suggests a potential auditory mechanism by which a fly could land and it is this mechanism that the project ultimately intends to explore.

Characterisation of Electroactive Foams as Active Sensors and Actuators

Our research has shown that the auditory neurones of some insects can act as sensors and actuators operating at the nano- and pico-scale levels. This neuronal motility contributes to the exquisite sensitivity and accuracy of auditory systems. This project aims at initiating the search for technological analogues that would exhibit such sensor and actuator dual function. This will first involve the experimental investigation of the mechanical behaviour of polypropylene thin films at the nanoscale, and will establish whether such films, if scaled down, could constitute suitable substrates for the development of nanoscale actuators. If so, micrometer size smart probes based of polymer foams could also to be used to further study the behaviour of dual biosensor / actuators such as auditory neurones.

School of Biological Sciences
Woodland Road
University of Bristol