The recently published Unweaving the Rainbow by Richard Dawkins is a treasure-house of scientific odds and ends. Dawkins's chair at Oxford University is sufficiently post-modern to seem like the creation of an Umberto Eco or a Malcolm Bradbury.
Nonetheless, the Simonyi Professor of the Public Understanding of Science takes his duties seriously, and has become famous and very controversial for debunking God. He is on safer ground, however, when it comes to insects' ears, which apparently are not barometers, but anemometers.
The function of an ear is to detect the tiny oscillations of the air we know as sound. These can be looked at as small variations in the barometric pressure, as the air is alternately compressed and then released by whatever makes the sound, the ripples of disturbance then propagating outwards from the source in all directions.
The needle of a tiny, very sensitive barometer located anywhere in the vicinity would swing up and down in phase with the sound waves passing by, and this is exactly how a human ear behaves. The eardrum moves in and out with the tiny pressure changes, its movement is relayed by nerve connections to the brain, and the brain can then tell whether a high-pitched sound or a low-pitched sound vibrates the drum.
But insect ears are little anemometers. Rather than being regarded as tiny changes in the atmospheric pressure, sound waves can be thought of as little winds, the air oscillating to and fro in response to those changes in the pressure; the air moves in one direction as the pressure rises, and then returns again shortly after the pressure falls.
While our "barometer" ears have a membrane stretched over a confined space, insect "anemometer" ears have either a hair or a membrane in a chamber open to the atmosphere; in either case it is blown back and forth by the rhythmic movement of the air.
A side-effect of this arrangement for the insect is that it is very easy for it to detect the direction from which any sound may come. As Dawkins puts it, thinking perhaps of some meteorologist of his acquaintance, "Any fool with a weathervane can distinguish a north wind from an east wind."
But for us this refinement is more difficult. We with our "barometer" ears have to calculate the direction of a sound by comparing the reports of two ears; the brain compares the relative loudness of the sounds, and in a separate exercise compares the relative times of arrival of the sounds at each ear, and then estimates where the sound is coming from.