Finding each other: Achroia grisella’s oddly simple directional ear

Achroia grisella is a pyraloid moth capable of directional hearing. It is generally accepted that hearing arose in moths to avoid bats, their main predators, and, in this particular case, was later repurposed into a rudimentary mating tool. The male emits ultrasonic signals of a wavelength several times larger than the distance between their tympana during its mating process. Notwithstanding, the female moths are capable of somewhat efficiently reaching the males after some zigzagging. Another remarkable feature is the simple structure of the ear, consisting of a cluster of just four receptor cells attached directly to the tympanic membrane. Moths with only one functioning ear have been seen to reach the males still, which implies the tympanum structure itself must confer the moth with monoaural directionality. A model is developed in COMSOL to explain the behavior of the moth ear. The complexity of the model is increased by improving the resemblance to the natural one, starting from a circular plate and progressing to, eventually, a damped elliptical plate with two different thicknesses and an attached point mass.