Directional Hearing in Insects and Other Small Animals: The Physics of Pressure-Difference Receiving Ears

Abstract

In many animals the eardrums receive sounds at both the external and internal surfaces, and the eardrums are responding to the vectorial difference between the external and internal sound pressures. Eardrum motion is a necessary part of the sensory transduction process, and it correlates strongly with ear directionality. The internal sound pressure acting on the eardrum can be measured by first calibrating the eardrum vibration with sound acting only at its external surface and then using measured vibrations of the eardrum for estimating the amplitude and phase of the sound(s) acting at its inner surface.

This procedure has been used in bushcrickets, grasshoppers, crickets, and budgerigars. The ears of many bushcrickets are located in the thin forelegs, but they are driven mainly by sound arriving at the inner surface of the eardrums from horn-shaped hearing trumpets that open at the lateral sides of the body. The eardrums of grasshoppers and budgerigars receive sound both at their external surfaces and at their internal surfaces, which are connected to the other ear by internal air spaces. This is also the case in crickets, but here the ears also receive sounds from two spiracular openings, which connect the inner surfaces of the eardrums with the air outside the animal. The transmission properties of these pathways have been measured, and the expected directionality has been calculated. Excellent agreements have been found between the calculated and the measured directionalities.