The Mechanics of Insect Audition: Characterisation Modelling and Application

The sense of hearing is one of the most widespread across the different species of animals in the world. Animals use hearing in communication, to listen for danger and to help find lunch. The frequencies of sound used can vary an enormous amount, from very low frequency detection (infrasound) in fish, to the extremely high frequencies used by bats to echolocate and hunt for prey (ultrasound). Of course humans also have a sense of hearing, ranging from low frequencies up to about 20 kHz, although as we get older, our ability to hear higher frequencies degrades. However, through our own ingenuity humans have learned to generate, detect and use ultrasound (frequencies above our frequency range). We use this in many different applications, including medical imaging, cleaning, material analysis and non-destructive testing. It was only by creating such ultrasound devices that people discovered that bats were using ultrasound to identify and chase insects, and that many insects had ears tuned to listen out for the hunting bats to try and escape becoming a meal. Recently, engineers have started to examine the way bats use ultrasound. This is because the bats can achieve far greater resolution and sensitivity than any human built ultrasound system. The engineers hope to be able to improve their artificial systems by working out what techniques the bats employ. Whilst we know a lot about the ultrasound signals the bats use, we know comparatively little about the hearing systems of the bat's prey; the insects. Many studies have shown us which insects are sensitive to ultrasound, for example by looking at the insect's behaviour when ultrasound is played back to it. And from that, eardrum-like structures in ultrasound sensitive insects were discovered. The performance of some insect ears has also been described using various techniques, including very hi-tech solutions such as laser interferometry (where a laser is used to measure the motion of the insect's eardrum in response to sound). However, the actual mechanical operation of the structures within the ears of these insects, and so our understanding of how they receive ultrasound and translate that to vibrations the nerve cells can detect is very poor. This new research will use a combination of engineering approaches to understand how the ultrasound sensitive ears of insects work. The mechanical motions of different structures in the ears will be measured, with their size, shape and material properties characterised. To do this several techniques will be used including laser interferometry and atomic force microscopy (AFM). An AFM images surfaces by touch, rather than light. It uses a very small, atomically sharp, tip that is dragged, or tapped, across the surface of an object. A record is made of how much this tip goes up and down allowing us to make a surface image. AFM's can be sensitive enough to map the atoms on the surface of a material. As well as imaging, an AFM tip can be pushed into a surface, allowing us to measure how soft or hard it is. Using this technique it's possible to map the stiffness of a material down to nanometre scales. Once all this new information is collected it will be used to help create computer models of the ear structures. We can compare the models with the actual motions we measure, helping us to understand what is happening in the ear. From this, the models provide us with a tool to explore the capabilities of other eardrums, and further our understanding of the different ear capabilities relating to their size, sensitivity and dynamic range. Finally, the new knowledge from this research has broader applications. Looking back to the engineers working on bat ultrasound signals, this research will show us how the ears that have evolved to detect the bat's calls operate. It may then help engineers striving to improve artificial ultrasound sensor systems across many different fields such as medicine, material science and engineering.

The proposed research will investigate the mechanical processes at work in the tympanal auditory organs of insects. The structure, material properties and mechanics of these organs will be investigated from sub-nanometre to micrometre length scales. Locusts are the model systems of choice for these investigations as they are available to be used in the applicant's laboratory; they are amenable to the planned experimental procedures and, despite their apparent structural simplicity, display many interesting functional attributes. In addition, further insect species chosen specifically for their known sensitivity to ultrasound will be investigated, including the night-flying tiger beetle, moth, praying mantis, and lacewing. The initial work will be focussed on the installation and combination of hearing organ stimuli with the existing microscanning laser vibrometer system, to investigate the organ's response to both static and dynamic stimuli. This will then allow the evaluation of the operating boundaries of the tympanal hearing systems, and the characterisation of the functionality of the ultrasound sensitive systems not previously examined using this experimental procedure. Further, the mechanical response of the internal structures within the tympanal systems will also be analysed. Structural data on all the tympanal systems will be gathered using optical, scanning electron, and atomic force microscopy. In addition to dimensional characterisation, atomic force microscopy will also be used to assess material properties of the structures, for example producing maps of Young's Modulus across the constituent parts of the tympanal ears. Finally, the structural and mechanical data will be combined to create finite element models of the structures present within the tympanal membrane systems. These models will be compared with experimental response data, allowing the analysis of both the operation of the tympanal ears and the capabilities of the modelling procedure.

There are three groups of beneficiaries for this research proposal. The first group are those currently undertaking fundamental work in the fields of hearing and sensory systems. The second group of beneficiaries are those in both academia and private sector industry that may use the outputs of this proposal as an opportunity to augment ultrasound based sensor and transducer systems. The final group are those in the general public whose lives may be positively affected, either by the improved understanding of how hearing and sensory systems work, or by the potential enhancements to ultrasound systems based on future developments stemming from this work. This research will have major impacts on the fundamental science of the mechanics of auditory systems. This will be followed by potential impacts through engineering development and commercial exploitation in ultrasound systems based on the fundamental research outputs. This exploitation will be targeted at UK industry through the applicant's research group's links with the UK National Centre for Research in NDE (Non-Destructive Evaluation), and the Scottish Executive sponsored Facility for Innovation and Research in Structural Testing (within Strathclyde). Thus, opportunities will be available to increase the UK's economic performance, building on the fundamental research within this proposal to develop applied research either with, or disseminated to, UK engineering and industry. The potential enhancement of future ultrasound systems from this exploitation could then have wide ranging impacts on various areas of society. Ultrasound systems are used in many different sectors, including medicine, engineering and science. Therefore, the work has the long term potential to enhance quality of life, health and the country's economic performance. The fundamental knowledge gained on hearing systems during the project will be available to other academics, including medical researchers, within 5 years of the project's start. The research and development in ultrasound engineering will follow on from the project, being realised 5-10 years from the project's start. Therefore, commercial exploitation from the fundamental, and then applied, research, could follow on 10+ years after the project start date. Further to this, the staff working on the project will develop a variety of research and professional skills, from project management, presentation and public outreach skills, to improved analytical and experimental skills in the areas of biological microscopy and mathematical modelling. The proposed project will engage all three groups of potential beneficiaries through its lifetime, and beyond. In addition to publication in high quality scientific journals, the research will be publicised across a wider academic base, and also to industry, public bodies and the general public. This will take place through various outlets including public talks, media interviews and publication in non-specialist magazines. The university's own publicity organization will also be utilised, for example through generation of press releases to the media. Industry will be targeted through meetings, seminars and symposia organised both within the UK National Centre for Research in Non-Destructive Evaluation, and the Facility for Innovation and Research in Structural Testing, plus any further related industrial outreach events. The applicant has experience in press interviews, writing for none specialist publications, and presenting to the general public. The University of Strathclyde has media training specifically for academics, which the applicant will use to hone and refresh their skills, and research staff will be encouraged to attend, with a view to also starting to interact with the media and public.