Are there active hearing processes in locust ears?

Poster Session held on Monday 29th June 2009 at the Annual Main Meeting of the Society of Experimental Biology, 28th June - 1st July, Glasgow, UK. The locust tympanal organ is a sophisticated example of insect hearing. The tympanal membrane has previously been shown to analyse the frequency of incident sound, such that sound energy is funnelled into waves of motion across the membrane, travelling to frequency specific neurone groups. In vertebrates Otoacoustic emissions have been shown to indicate active hearing processes. These sound emissions are produced as a by-product in the ears of vertebrates, and can be spontaneous or evoked by acoustic stimulus. The evoked emissions, known as distortion-products, are found in active auditory systems when the organ is stimulated with two sounds of different frequencies, f1 and f2. In some insects, including locusts, previous studies have found evidence of distortion-products, but the experimental approach raises questions as to whether the results are actually driven by active hearing processes. In these previous experiments a closed-air system was used with the sound stimulus very close to the organ. The experiments have been revised, altering the experimental set-up to an open system, with appropriate sound pressure levels, such as the ear would expect in a natural setting. A laser vibrometer records the deflection of the locust tympanal membrane at specific points where the mechanoreceptor neurones attach. This allows the membrane to be measured directly for the presence of distortion-products, at the position of the neurones which are believed to be actively generating membrane sound emissions. The measurement, or lack thereof, of distortion-products in the motion of the tympanal membrane is discussed and compared with previous studies