Active amplification in insect ears: mechanics, models and molecules
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REVIEW
Active amplification in insect ears: mechanics, models and molecules Natasha Mhatre
Received: 27 August 2014 / Revised: 15 November 2014 / Accepted: 17 November 2014 / Published online: 11 December 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Active amplification in auditory systems is a unique and sophisticated mechanism that expends energy in amplifying the mechanical input to the auditory system, to increase its sensitivity and acuity. Although known for decades from vertebrates, active auditory amplification was only discovered in insects relatively recently. It was first discovered from two dipterans, mosquitoes and flies, who hear with their light and compliant antennae; only recently has it been observed in the stiffer and heavier tympanal ears of an orthopteran. The discovery of active amplification in two distinct insect lineages with independently evolved ears, suggests that the trait may be ancestral, and other insects may possess it as well. This opens up extensive research possibilities in the field of acoustic communication, not just in auditory biophysics, but also in behaviour and neurobiology. The scope of this review is to establish benchmarks for identifying the presence of active amplification in an auditory system and to review the evidence we currently have from different insect ears. I also review some of the models that have been posited to explain the mechanism, both from vertebrates and insects and then review the current mechanical, neurobiological and genetic evidence for each of these models.
N. Mhatre (*) School of Biological Sciences, University of Bristol, Woodland road, Bristol BS8 1UG, UK e-mail: [email protected] Present Address: N. Mhatre Department of Biological Sciences, Integrative Behaviour and Neuroscience Group, University of Toronto Scarborough, 1265 Military Trail, Scarborough, ON M1C 1A4, Canada
Keywords Acoustic communication · Active hearing · Insect hearing · Active auditory amplification · Active mechanosensation Abbreviations BF Best frequency CAP Compound action potentials Iav Inactive IHC Inner hair cell JO Johnston’s organ OHC Outer hair cell Nan Nanchung NompC No mechanoreceptor potential C SRS Stimulus receiver structure TilB Touch insensitive larvae B TRP Transient receptor potential
Active versus passive sensing Insects use their ears for a range of functions: to find resources, such as mates or hosts to parasitize, but also to avoid becoming a resource, to avoid parasites and predators (reviewed in Gerhardt and Huber 2002). Auditory function is thus essential and under strong selection. Auditory systems are therefore often finely honed to respond to the most salient sounds. Research has uncovered many mechanisms that auditory systems use to ensure sensitivity. One broad class of adaptations are structural adaptations which use physical form to generate function. They include the resonance of field cricket tympana, which crickets exploit to be tuned to their conspecific song (Paton et al. 1977; Larsen et al. 1989).
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