How are flies able to change their flying direction quickly?

A fly, Musca domestica, has four wings joined to its body in the thorax region where the legs are also joined.

The flies fly by flapping their wings at a very fast rate ranging from 50 to more than 1000 flaps per second.

While the articulation of the two forewings actually propels the fly, rear wings are rather redundant or vestigial because they are not used for propelling.

But they also vibrate along with the forewings, however, in the vertical direction. Thus, they act as gyroscopes to inform the fly about the turns and tilts so that the forewings can effectively thrust the insect forward on a steady course.

The left and right forewings are highly synchronised for a steady flight. A fly, in flight, can make very sharp 90 degree turns in less than a millisecond, i.e. 1/1000 of a second. This is faster than the speed of a blink of a human eye. Such a manoeuvre is called a saccade and for this the fly has to generate enough of torque to overcome both the inertia of motion of itself and the viscous drag of the air.

Detailed high speed photography has revealed that during a saccade, a fly has also to negotiate the necessary banking or tilting to the side it turns.

This is accomplished by the rear wings while the fore wings actually manage the turn by differential beating. If, for example, the fly has to turn to left, the left wings momentarily stop with the right wings continuing to flap. This provides the necessary twisting torque in a very short time. It is now clear that the air friction is not very important and the inertia alone governs the fantastic feat of near instantaneous turning in air.

Source: The Hindu