Mechanically coupled ears for directional hearing in the parasitoid fly Ormia ochracea

An analysis is presented of the mechanical response to a sound field of the ears of the parasitoid fly Ormia ochracea. This animal shows a remarkable ability to detect the direction of an incident sound stimulus even though its acoustic sensory organs are in very close proximity to each other. This close proximity causes the arrival times of the sound pressures at the two ears to be less than 1 to 2 microseconds depending on the direction of propagation of the sound wave. The small differences in these two pressures must be processed by the animal in order to determine the incident direction of the sound. In this fly, the ears are so close together that they are actually joined by a cuticular structure which couples their motion mechanically and subsequently magnifies interaural differences. The use of a cuticular structure as a means to couple the ears to achieve directional sensitivity is novel and has not been reported in previous studies of directional hearing. An analytical model of the mechanical response of the ear to a sound stimulus is proposed which supports the claim that mechanical interaural coupling is the key to this animal's ability to localize sound sources. Predicted results for sound fields having a range of incident directions are presented and are found to agree very well with measurements.