Distortion-product otoacoustic emissions from the tympanic organ in two noctuoid moths

Distortion-product otoacoustic emissions were measured from the tympanic organ of Ascalapha odorata (Noctuidae, Erebinae) and Empyreuma affinis (=pugione) (Arctiidae, Ctenuchinae) during stimulation with two pure tones of different frequency (f1,f2) within a frequency range between 5 and 60 kHz. The cubic distortion-product that appears at a frequency of 2f1-f2 showed the largest amplitude and could be used to obtain "distortion audiograms". In such audiograms, the maximal distortion-product otoacoustic emission levels were measured for stimulus frequencies that correspond to lowest Al-cell thresholds in each of the two species. Newly emerged E. affinis showed very low levels of the 2f1-f2 distortion, most of which were within the noise range. In contrast to vertebrate distortion-product otoacoustic emissions, in neither of the species could an optimum f2/f1 ratio be easily defined, and small ratios were sufficient to induce the distortions. We conclude that the ears of the two moth species exhibit distinctly non-linear mechanical properties. Distortion-product otoacoustic emissions are a good non-invasive method for studying the spectral response characteristics of the mechanics of their auditory organs, despite the fact that their morphological characteristics are very different from those in vertebrates.