Houseflies see human movement in slow motion because their brains process visual information four times faster than ours.

The speed at which an animal perceives the world is determined by its "flicker fusion frequency"—the threshold at which individual flashes of light merge into a steady stream. While humans have a flicker fusion rate of approximately 60 hertz, the common housefly (Musca domestica) can reach rates as high as 250 hertz.A 2013 study published in Animal Behaviour by researchers from Trinity College Dublin confirmed that smaller animals with high metabolic rates perceive time more slowly. This is an evolutionary survival mechanism that allows flies to observe a predator's or swatter's trajectory with extreme precision. To a fly, a human hand moving at high speed appears to glide through molasses.This visual advantage is enhanced by specialized structures called "halteres"—small, knob-like organs behind the wings that function like gyroscopes. They provide instant feedback on the fly's body position during rapid maneuvers, enabling split-second course corrections.The energy cost of such high-speed vision is substantial. Because flies have high metabolic rates, they must consume food constantly to power their hyperactive nervous systems. This trade-off allows them to react to a threat in just 30 to 50 milliseconds—far faster than a human eye can blink.