Flies can react to an object they see and change direction in less than 100 milliseconds.
Using high-resolution, high-speed digital imaging of fruit flies (Drosophila melanogaster) faced with a looming swatter, the secret to a fly's evasive maneuvering has been discovered. Long before the fly leaps, its tiny brain calculates the location of the impending threat, comes up with an escape plan, and places its legs in an optimal position to hop out of the way in the opposite direction. All of this action takes place within about 100 milliseconds after the fly first spots the swatter.
This illustrates how rapidly the fly's brain can process sensory information into an appropriate motor response.
For example, the videos showed that if the descending swatter--actually, a 14-centimeter-diameter black disk, dropping at a 50-degree angle toward a fly standing at the center of a small platform--comes from in front of the fly, the fly moves its middle legs forward and leans back, then raises and extends its legs to push off backward. When the threat comes from the back, however, the fly (which has a nearly 360-degree field of view and can see behind itself) moves its middle legs a tiny bit backward. With a threat from the side, the fly keeps its middle legs stationary but leans its whole body in the opposite direction before it jumps.