A hover fly prior to release. Photo credit: R. Goulard and S. Viollet.

A hover fly prior to release. Photo credit: R. Goulard and S. Viollet.

The screaming on most rollercoasters begins almost as soon as anticipation of the stomach-churning descent begins. Even in the dark we know that we are plummeting from the instant we tip over the summit, and the secret of the thrill lies deep in gravity sensors in our inner ears and intestines. As soon as the descent is launched and we begin free falling, the effect of gravity on the sensors is eliminated, producing the familiar stomach-churning sensation. However, Stéphane Viollet, from the CNRS and Aix-Marseille University, France, explains that the feedback from gravity sensors can sometimes be misleading and in some circumstances they may even fail if a manoeuvre is particularly extreme, saturating the receptors so that they no longer respond. Knowing that hover flies are extraordinarily agile and routinely perform aggressive high acceleration manoeuvres, Viollet and colleagues Roman Goulard and Jean-Louis Vercher wondered whether the insects rely on gravity-sensing organs to detect when the world has fallen away from beneath their feet.

Designing a gravity-defying joy-ride in which the unsuspecting flies were dropped instantaneously from an electromagnet, Viollet and Goulard were then able to simulate free-fall conditions to find out how swiftly the insects responded to the loss of the sensation of gravity and attempted to fly. However, knowing that flies also rely on vision and the sensation of air flowing over mechanosensory hairs on their bodies to determine their orientation, Viollet and Goulard also messed with their view of the world by releasing the flies in three different surroundings: pitch dark, a uniform white box illuminated from above and a box lined with horizontal stripes that should intensify the visual sensation as they fell.

Having filmed the plummeting flies and noted when the insects initiated a last-ditch effort to pull out of the dive, Viollet and Goulard realised that the flies that fell in the dark rarely, if ever, began beating their wings in time to rescue themselves from a crash. ‘After 200 ms in every condition it is impossible for the fly to stop the fall’, says Viollet; so the free-falling flies seemed to be unaware that they were falling until they had passed the point of no return, and even then they had no sense of which direction they were moving and often flew into the floor or wall, instead of soaring to safety. They clearly had no gravity sensors – accelerometers – and must be relying on the sensation of air flow to rectify their fall.

However, as Viollet and Goulard gradually reintroduced visual cues, the flies seemed to do better: they triggered wing beats earlier as they fell in the white box, and when the duo installed the stripy wallpaper the majority of the flies triggered flight and avoided a crash within 150 ms of release. The flies were using vision in addition to sensing airflow to determine their orientation. The two scientists are now eager to understand how the tiny aviators integrate all of this information in their brains, as they hope to use the information to design better autopilots. ‘Every airplane or drone is equipped with accelerometers because they are vital for the vehicles to measure their orientation… but it seems that flies do not need that… so it means that there are tricks that flying insects are using that don't use accelerometers to stabilise their flight’, says Viollet, adding, ‘For us it is a complete breakthrough in the design of autopilot systems’.

To crash or not to crash: how do hoverflies cope with free-fall situations and weightlessness?
J. Exp. Biol.