Insects swim through the air
High-speed filming of everybody’s favorite fruit fly, Drosophila melanogaster, have shown that the insects paddle their way through air much as many aquatic animals do to achieve fast forward speeds. Previously, people believed that most insects used lift forces instead of drag forces.
There are two ways to move yourself forward if you fly. You either use a drag force, in which you move your wings forward and backward, pushing on the air, relying on the drag force to propel you forward. Or you can use lift forces which are perpendicular to that motion, essentially pushing air down instead of backwards, and then tilt your whole body so that a part of that downward force pushes backward.
Physicists made 140 videos of fruit flies flying forward and selected out the 16 in which the wings stayed in a horizontal plane. By examining the video, they discovered that the flies move their wings forward at a shallow angle, effectively slicing through the air, and then pushed back at a steeper angle to create drag forces which pushed the flies forward.
Although pretty much all flight patterns are more complicated than just moving backward and forward, they researchers found that it was this paddling motion that was responsible for forward propulsion in many cases. If done just right, the paddling can also provide sufficient lift to keep the fly afloat.
Curiously, this finding has significance for evolutionary arguments about the origins of insect flight. Because many aquatic animals use drag forces and insects were thought to use lift forces, many people thought that the flight mechanisms were quite different and so there was unlikely to be an evolutionary path from swimming to flying. This find brings that common assumption into question and suggests that flying insects could well have evolved from water-dwelling creatures.
A video from the Cornell group about analyzing fruit fly motion.