Just relying on their eyes is not enough for the fruitfly Drosophila when zooming to that pitiful last banana on your counter. Wind-sensing antennae allow fruitflies to fly at constant speed, even when winds change.
Characteristically, fruitflies straight flight bouts are punctuated by quick changes in direction. Flies tend fly straight at a constant speed, even when wind speeds change. This suggests that flies actively regulate their groundspeed. The visual information of just seeing your kitchen pass by isn’t fast enough for the fly. 50 to 100 milliseconds have passed by the time the image reaches the fly’s brain and is processed so that it can respond, which roughly equates to 10 to 20 wing beats between seeing and reacting.
Sensors on the antenna of flies measure the speed of passing air with a delay of only about 10 milliseconds. In a paper published this week in PNAS, researchers let flies with clipped antennae fly in a wind tunnel, and compared how they fared in short and rapid gusts of wind with flies with intact wings. Flies react to wind in an unexpected way, in the same direction as air drag – in headwind gusts, flies slow down, while in tailwind gusts, they fly faster. But afterwards, flies with intact antennae quickly return back to a set groundspeed. Flies without antennae return to their groundspeed more slowly, and in steady wind, the groundspeed of flies without antennae changes greatly.
The researchers show that airspeed measured by the antennae is combined with what the fly sees to achieve a steady groundspeed. Visual information is a baseline to regulate groundspeed to a specific velocity, but it has a long delay. So if windspeed changes quickly, the visual information is not fast enough to adapt flight speed rapidly and reliable: By the time the fly is able to react, the wind might have already changed again, and the adaptation overshoots. Airspeed measurement by antennae has a short delay, and gives a robust input that makes speed changes less variable. Combining airspeed with visual information allows flies to fly at a stable groundspeed, and rapidly react to wind speed changes.
Feeling the air and seeing the kitchen pass by, flies adapt their speed to changing winds, helping them to home in on your lonely banana.
If you would like to give your banana a better fate than being fly fodder, I suggest smitten kitchen’s recipe for chocolate banana bread.