When it first notices an approaching threat, a fly's body might be in any sort of posture depending on what it was doing at the time, like grooming, feeding, walking, or courting. Our experiments showed that the fly somehow 'knows' whether it needs to make large or small postural changes to reach the correct preflight posture.

It is best not to swat at the fly's starting position, but rather to aim a bit forward of that to anticipate where the fly is going to jump when it first sees your swatter.

Michael Dickinson Quotes