Biomechanical investigation of the locust ear with 3D laser Doppler vibrometry

One of the models of insect tympanal hearing, the desert locust Schistocerca gregaria, has already been the subject of numerous studies investigating membrane mechanics, neurophysiology, and ethology over the past few years in order to better understand insect hearing. Acquiring insight into the biomechanics of insect hearing is to study how their ears move; the impact of tympanal structure on frequency analysis in the locust has already been investigated using laser Doppler vibrometry to record membrane displacement (e.g. Windmill et al 2005). Thanks to this approach it is known that, in locusts, sounds of given frequencies will generate travelling waves across the tympanum that propagate to different locations depending on the frequency of the initial stimulus. Locust ears are characterized by several spatially segregated groups of mechanosensors which are tuned to different ranges of frequencies, from 1-12 kHz in the folded body region (FB), to 12-30 kHz around the pyriform vesicle (PV). These travelling waves thus allow for the incoming stimulus to be concentrated in the region of the tympanum best tuned to its frequency, a phenomenon which can be regarded as a first, mechanical step in the process of frequency analysis. The research addressed in this work applies recent technological advances in 3D micro-scanning laser Doppler vibrometry to measure membrane displacement. Thus, in addition to the travelling waves identified through vibration measurements in only one dimension by previous studies, this work has for the first time detected and identified membrane oscillations in three dimensions simultaneously. The results allow us to improve our understanding of the impact of membrane mechanics on frequency analysis in the locust ear