1 3 J Comp Physiol A DOI 10.1007/s00359-014-0914-2 ORIGINAL PAPER Mechanical basis of otoacoustic emissions in tympanal hearing organs Doreen Möckel · Manuela Nowotny · Manfred Kössl Received: 29 January 2014 / Revised: 14 April 2014 / Accepted: 16 April 2014 © Springer-Verlag Berlin Heidelberg 2014 Keywords DPOAE · Laser Doppler vibrometry · Locust · Tympanum · Insect Abbreviations DPOAE Distortion–product otoacoustic emission LDV Laser Doppler vibrometry SPL Sound pressure level Introduction Insects with tympanal organs produce sensitive distortion– product otoacoustic emissions (DPOAEs), as shown in locusts (Kössl and Boyan 1998; Möckel et al. 2007, 2012), moths (Kössl and Coro 2006; Kössl et al. 2007) and bush- crickets (Möckel et al. 2011), that in mammals are used as indicator for nonlinear cochlear amplification. Character- istics of insect DPOAEs are largely comparable to those reported from vertebrate ears. They appear during simul- taneous stimulation with two pure tones (f 1 < f 2 ) as addi- tional spectral peaks at frequencies of nf 1 - (n - 1)f 2 and nf 2 - (n - 1)f 1 . The 2f 1 - f 2 emission is most prominent, as it is evoked by lowest stimulus levels, with the largest amplitude reached at frequencies of high auditory sensitiv- ity. In moths and locusts, the 2f 1 - f 2 DPOAEs are elicited already at stimuli levels near the species-specific auditory threshold (Kössl et al. 2008). Insect DPOAEs are highly vulnerable to manipulations that interfere with the animal’s physiological state, such as hypoxia (Kössl and Boyan 1998) or ethyl ether (Kössl et al. 2007). First evidence that the auditory mechanoreceptors and specifically the dynein– tubulin system within could be relevant for DPOAE gen- eration came from selective exclusion of mechanoreceptors via mechanical lesions (on locusts, Möckel et al. 2007), the effects of a neuroactive insecticide (on bushcrickets, Abstract Tympanal hearing organs of insects emit dis- tortion–product otoacoustic emissions (DPOAEs), which in mammals are used as indicator for nonlinear cochlear amplification, and which are highly vulnerable to manipu- lations interfering with the animal’s physiological state. Although in previous studies, evidence was provided for the involvement of auditory mechanoreceptors, the source of DPOAE generation and possible active mechanisms in tympanal organs remained unknown. Using laser Dop- pler vibrometry in the locust ear, we show that DPOAEs mechanically emerge at the tympanum region where the auditory mechanoreceptors are attached. Those emission- coupled vibrations differed remarkably from tympanum waves evoked by external pure tones of the same frequency, in terms of wave propagation, energy distribution, and loca- tion of amplitude maxima. Selective inactivation of the auditory receptor cells by mechanical lesions did not affect the tympanum’s response to external pure tones, but abol- ished the emission’s displacement amplitude peak. These findings provide evidence that tympanal auditory recep- tors, comparable to the situation in mammals, comprise the required nonlinear response characteristics, which during two-tone stimulation lead to additional, highly localized deflections of the tympanum. Electronic supplementary material The online version of this article (doi:10.1007/s00359-014-0914-2) contains supplementary material, which is available to authorized users. D. Möckel (*) · M. Nowotny · M. Kössl Institut für Zellbiologie und Neurowissenschaft, J. W. Goethe-Universität, Biologicum A, Max-von-Laue-Straße 13, 60438 Frankfurt am Main, Germany e-mail: moeckel@bio.uni-frankfurt.de