A kind of ear new to science, a swiveling structure that picks up odors as well as sounds, enables fruit flies to sense sounds.
“It’s rather elegant,” says Daniel Robert of the University of Zurich in Switzerland, where he discovered the mechanism with colleague Martin Göpfert. Their equipment capable of measuring nanoscale vibrations reveals that in response to sound, a Drosophila antenna twists like a key in a lock. Robert and Göpfert report their findings in the June 21 Nature.
As far back as the 1960s, researchers realized that fruit flies use their feathery antennae to detect airborne sounds, including entomological love songs. A courting male serenades a female with wing vibrations. When researchers clipped a little of a female’s antennae, she seemed less responsive to the male’s charms. The feathery upright stalk of the antenna, or arista, extends barely 300 microns in length, so only recently have measurements of tiny antennae movements been possible.
Robert and Göpfert used a laser-based motion detector to track minute vibrations in the arista and the two-part oblong base of a Drosophila antenna. From the bigger part of the base, a tiny hook extends into the smaller section.
Scientists had previously assumed that the arista flexes to pick up the air motions of sound, according to Robert and Göpfert. Instead, they found that the arista oscillates as a whole and sets the larger part of the base moving. This mass pivots about its long axis. As it does so, sensory cells at its hooklike end pick up the motion.
The system is efficient. The hook “is not wobbling in all directions,” says Robert. “It’s really well balanced.”
The basal mass holds hundreds of tiny olfactory sensors. Yet hearing by swiveling doesn’t seem to compromise smelling, Robert marvels. The happy partnership of these senses enables a fly to perceive complex suites of signals, including those that presage mating. “There are all sorts of things going on: smelling, tasting, touching,” he says.
Since discovering this mechanism in Drosophila, Robert and Göpfert have found similar structures in a sampling of related flies. Robert predicts that swivel hearing will turn out to be widespread in this group.
“This is exciting for us,” says Daniel Eberl of the University of Iowa in Iowa City, where he and colleagues study the nervous system behind Drosophila hearing. For decades, he says, researchers have conjectured about the basic principles of the system. Knowing precisely what mechanical stimuli trigger nerve cells will help his team decode the fly’s sensory response.
Robert predicts that the newly revealed details of the fruit fly sensory system will inspire fresh perspectives on hearing. Most vertebrates hear with some variation of a vibrating eardrum in their heads, but insects offer a much richer variety of design, says Robert. They can deploy ears just about everywhere: wings, legs, bellies.
For us vertebrates, how strange it seems to hear with a nose. “Flies did not turn their nose into an ear,” Robert points out. “They turn their nose to hear.”