Flies are hard to catch. Hard to even touch. After all, they’ve been biologically wired over millions of years of evolution to evade your swat. So how do they do it, and how can you take advantage of knowing their behavior? 

There are three primary ways the housefly evades your hand. First: vision. Their compound eyes give them a wide field of vision, and allow them to detect motion efficiently. Each compound eye is made up of thousands of tiny lenses which, combined, give the fly a panoramic view of its surroundings. While a housefly may not see fine details as sharply as we do, these multiple lenses enable it to detect motion from almost any angle. Even the smallest shift in light or shadow can trigger the housefly’s alarm response, prompting it to take off before a predator can land a hit. 

In addition, house flies perceive time differently from humans. Humans are able to distinguish about 60 flashes of light per second before light appears to be a continuous stream. Houseflies, however, can discern more than 200. This speed at which our photoreceptors, light-detecting cells, process light relates to how we perceive time. Houseflies can react to external stimuli in as little as 20 milliseconds—for comparison, the average human reaction time is about 250 milliseconds. Flies are able to react much faster than we do because of this difference; as fast as our hands might seem to us, they move at a mammoth’s pace compared to what the housefly perceives. 

In addition, the hairs covering their body are incredibly efficient at detecting air movement, meaning that the fly can detect the air your hand or fly swatter pushes towards it even before it sees that you’re coming for it. Little clubs below its wings, called halteres, help the housefly process its orientation in space to fly erratically without falling to the ground. 

The erratic flight patterns you observe when a fly is escaping are highly adaptive maneuvers that combine rapid wingbeats, precise haltere feedback, and instantaneous environmental cues. When threatened, a housefly can quickly change direction mid-flight, zigzagging in short bursts to confuse predators. These unpredictable movements often make it seem as if the fly disappears and reappears in another spot in the blink of an eye. By continually altering its trajectory, the fly reduces the likelihood that a pursuer can anticipate where it will be next. 

So houseflies are built to avoid us. How can we take advantage of this knowledge? Sanjay Sane, a fly researcher at the National Centre for Biological Sciences at the Tata Institute of Fundamental Research, says we can use their slow time perception and motion-oriented vision to our benefit. By moving slowly at a static pace and without jerky movements, the fly will essentially perceive us as unmoving. Once we are close enough, even the fly’s quick reaction time and speed won’t be enough to avoid us.