As a fruit fly twists and dives through the air, two major environmental forces act on its gyrating body: inertia and friction. While the insect's inertia is entirely down to its body mass, it was less clear how significant air friction is on the tiny aeronaut. Fritz-Olaf Lehmann explains that the insect's light wings contribute little to the insect's mass, and so virtually nothing to their inertia. However, they comprise a significant surface area. Could they contribute significantly to the friction on the tiny aviator?Curious to know how friction contributes to a fly's manoeuvrability, Lehmann and his postdoc Thomas Hesselberg calculated the damping coefficient due to frictional forces acting on a fly's flapping wing during a saccadic turn and found that it was 100 times greater than the friction acting on the body alone(). Far from being insignificant, friction appears to be a major force on the fly's activities and could help rapidly turning flies to stop. Hesselberg and Lehmann realised that instead of actively breaking, the sticky air could halt a rapidly turning fly. Following the simulations the pair decided to test frictional effects on the insect's flight in a controlled environment.
Tethering fruit flies inside a high-tech flight arena, Hesselberg and Lehmann tested the insects' responses to a simulation of the insect's view as it turned towards a moving target. Hesselberg designed a complicated feedback control system that forced the insect to steer as it flew is if approaching a moving object, feeding back the insect's behaviour into the simulation. By altering the simulated view, the pair could fool the insect's flight control system and trick it into behaving as if it were flying through thick and normal air, so that the team could measure the insect's steering accuracy to see how well it manoeuvred at high and low friction. The team found that the tethered fly coped reasonably well flying through most of the simulations,except when the damping was especially high or at low (normal) conditions;failing to turn when the damping was high, as if the air was too thick; and turning uncontrollably when the damping was low, as if normal air is too thin to stop the rotation.
One reason that has been suggested for control failure when the insect is flying in low damping (normal) air is that the fruit fly's visual system cannot respond fast enough to rapid turns, forcing a freely flying insect to rely on other sensory systems, such as the gyroscopic halteres. However, when the flies are tethered, the halteres are no longer able to supply information for precise flight control. With only their eyes to guide them, tethered insects have to rely on exceptionally precise wing beat control, between 1-2°, to control turning; which is beyond even these aviators' talents. However, Lehmann and Hesselberg found that the flies could successfully control flight when they increased the damping above normal levels to a point where the flies only needed a wing beat accuracy of 3-4°, suggesting that flies could manoeuvre accurately using their eyes alone if the sensory information was delivered as fast as information from the halteres. All of which suggests that fruit flies are flying on the edge, depending on every control and sensory system available to them.
