Abstract

Frequency analysis in the mammalian cochlea depends on the propagation of frequency information in the form of a
travelling wave (TW) across tonotopically arranged auditory sensilla. TWs have been directly observed in the basilar papilla
of birds and the ears of bush-crickets (Insecta: Orthoptera) and have also been indirectly inferred in the hearing organs of some reptiles and frogs. Existing experimental approaches to measure TW function in tetrapods and bushcrickets
are inherently invasive, compromising the fine-scale mechanics of each system. Located in the forelegs, the bushcricket
ear exhibits outer, middle and inner components; the inner ear containing tonotopically arranged auditory
sensilla within a fluid-filled cavity, and externally protected by the leg cuticle. Here, we report bush-crickets with
transparent ear cuticles as potential model species for direct, non-invasive measuring of TWs and tonotopy. Using laser
Doppler vibrometry and spectroscopy, we show that increased transmittance of light through the ear cuticle allows for
effective non-invasive measurements of TWs and frequency mapping. More transparent cuticles allow several properties
of TWs to be precisely recovered and measured in vivo from intact specimens. Our approach provides an innovative, noninvasive alternative to measure the natural motion of the sensillia-bearing surface embedded in the intact inner ear fluid.