Explaining the monoaural directional hearing of the moth Achroia grisella

Achroia grisella (Fabricius, 1794) (Lepidoptera: Pyralidae) is a pyralid moth with two ears in its abdomen that it uses for detecting mates and predators. Despite no connection between the two ears having been found and no other elements having been observed through X-ray scans of the moth, it seems to be capable of directional hearing with just one ear when one of them is damaged. It is therefore suspected that the morphology of the eardrum can provide directional cues for sound localization. Here, we use finite element modelling software COMSOL to model a simplified version of the eardrum, an elliptical plate with two sections of different thicknesses and a mass load at the centre of the thin section, to try to determine if the morphology of the ear is responsible for the moth’s monoaural directional hearing. Results indicate that the resonance mode and directionality response of the elliptical plate with two thicknesses and a mass load match that of the moth closely and provide an enhanced response to sounds coming from the front of the moth. Damping is also considered in the resonant mode, and it is observed to improve the resemblance of the simulation to real moth ear measurements.