Image caption The hairs are just 100 millionths of a metre long - only as long as human hairs are wide

Tiny hairs that cover bats' wings seem to act as a complex airspeed sensor, researchers report in Proceedings of the National Academy of Science.

Experiments showed that bats' brains distinguished between airflow direction when air flowed over them at low speed.

However, with the hairs removed, the bats executed fewer tight turns and flew at higher speeds.

The researchers suggest the hairs allow fine control over aerodynamics as airflow changes during manoeuvres.

The hairs are exceptionally small - only as long as human hairs are wide, and just billionths of a metre thick.

Tiny hair-like structures on many insects are known to respond to stimulation, but their role in aerodynamic control remains a subject of study. At issue is the complex manoeuvring that is required to generate enough lift to keep insects - or, indeed, bats - aloft.

"The presence of these hairs on the wings of the bat was described in physiological studies over a hundred years ago... but they weren't really followed up," said Cynthia Moss, the University of Maryland cognitive scientist who led the study.

"These are not the same hairs as those you have on your arm," she told BBC News. "They're very tiny and very stiff, and that makes them good candidates for carrying detailed information about airflow across the wing."

Bubble power

Professor Moss and her team started by measuring the responses in bats' brains as the hairs were stimulated by controlled, directed blasts of air. The responses were distinct for airflows of differing directions, suggesting the bats can distinguish between them.

The bats were then presented with an obstacle course, and their movements captured and mapped out using video cameras. The hairs were then removed from the wings, and the experiment repeated.

Bats without the hairs executed fewer tight turns, and flew on average much faster. Combined with the result that the hairs on the wings' back edges were most sensitive to airflow opposite to the direction of travel, there is a good explanation for just what the hairs may be controlling.

Image caption The hairs are particularly stiff, making them more sensitive to small changes in airflow

Insects make use of what is known as a "leading-edge vortex" - at high angles, the airflow at the wings' front edges actually separates from the wing surface, creating a "bubble" of low pressure over the wing's area.

This bubble rejoins the wing at its back edge, where airflow is opposite to the direction of travel.

The low-pressure region above the wing creates a great deal of extra lift, which is particularly useful when the animals are flying at low speeds and executing tighter turns.

However, these vortices are unstable; slight changes in airflow can disrupt them, removing the lift that they provide.

In 2008, Anders Hedenstrom of Lund University in Sweden and his team showed in an article in Science that despite being much larger animals, bats too use leading-edge vortices for extra lift at slow speeds.

"It's a nice experimental demonstration of the function of these hairs, since depilation changed flight behaviour. But there is a long way to go before we know in detail what these hairs do," Professor Hedenstrom told BBC News.

"The hair cells of the present study were most sensitive to flow in the reverse direction, suggesting to me that they may play a function in sensing whether a leading-edge vortex is present or not, and helping the bat to control the wing movement to form a leading-edge vortex."

Professor Moss agreed that "there's certainly a lot more work to be done to get a clear picture of what's going on" with the hairs.

She and her team are hoping to carry out experiments watching bats' neural responses in mid-flight, as well as more detailed studies of how airflow changes across bats' wings as they fly.