A model of a fly, not a fly. Three states, flying straight, turning and resting, and a handful of numbers that say how long each lasts. The numbers are at the end, so you can argue with them.
Saccade
A fly does not curve. It flies a straight run, snaps through about ninety degrees in a twentieth of a second, and flies straight again. Inside a run it is predictable. The surprise is only ever when the next turn comes.
Wagner 1986, houseflies. Tammero & Dickinson 2002, fruit flies: 50 to 70 ms a turn.
The memoryless turn
Having flown straight for a while tells you nothing about whether it will keep doing so. The gap between turns is drawn from an exponential, the one distribution with exactly that property. That is why watching longer does not help.
Lag
Your swat lands where you aimed, a beat after you decided. With a lag of L against runs that average s, the fly has turned before the swat arrives 1 − e−L/s of the time: at 180 ms against a housefly, about 40%. That one line is the whole game.
It sees you coming
A resting fly shifts its legs to point its jump away from a looming swatter and is off the wall within a couple of hundred milliseconds, before your arm has moved. Here it notices a swat launched within thirty centimetres and leaves 150 ms later. Approach slowly, aim a little ahead of where it faces.
Card & Dickinson 2008.
The spook
A miss is a swish, and the fly goes frantic: a third faster and turning twice as often, for a couple of seconds, then back to its pattern. That one is from watching real ones, which take longer to settle than this. The trail turns warm while it lasts.
Rest spots
A fly in a room spends most of its time sitting, favours edges and corners, and goes back to the same ones. The oldest advice is about this, not about flight: wait for it to land. The escape reflex is what makes even that hard.
The mosquito
It turns less often and drifts at a third of the housefly's speed, so the same hand mostly wins the bet against it. That is why it is the easy one here, and why in life it is not: a mosquito is hard to see, not hard to hit.
The moth and the lamp
Away from a light a moth is erratic, all wander and no plan. Near one, a reflex that keeps its back to the brightest part of the sky, which for a few hundred million years was up, points it at the lamp instead, and the correction becomes an orbit. The lamp is a trap, and the trap is where to wait.
Fabian et al. 2024, which replaced the older guess about moths mistaking bulbs for the moon.
The swatter is not a point
A swat that lands within ten centimetres counts as a hit; that is the ring. The wall is two metres wide at 2.5 mm to a unit. The histogram below the wall is how far every swat missed by, yours in colour, everyone's in grey.
The daily
One round a day, the same for everyone: the date seeds the model, so every player's fly starts in the same place and would fly the same route if nobody swatted. The moment you swat, yours is yours, because a swat spooks it. One attempt, kept in this browser. The line of dots is your twenty swats in order and gives the fly away to nobody.
The crowd
Each swat goes to the wall as five numbers and two words: mode, insect, lag, how far it missed by, and in sniper mode how long you waited. The daily adds a day and a score. An opaque id in this browser lets later swats update earlier ones. That, and a count of page opens, is everything this site stores: no cookies, no analytics, no names, and nothing loaded from anyone else, not even a font.
Speeds are for a two-metre wall. The housefly's is on the low side of what they manage in a room, because a faster one on a wall this small spends its life avoiding the edges. Turn angles are drawn from a bell curve around the stated value, a turn's timing from an exponential with the stated mean. The rest, the edge-seeking, the length of a rest, is a guess with a plausible shape.
Not here: depth. A real fly leaves the wall, and a swat from the side is a different problem from one arriving flat. Not here either: wind from the swatter, or a fly that learns you are there. A hit leaves a mark and the next insect comes in from the edge a beat later. A round is twenty swats: long enough to see the pattern, short enough that the page ends rather than you having to. A challenge link carries the insect, the speed, the lag, your score and a first name if you type one; nothing about it is stored, so faking one fools exactly one friend. The tests under tests/ run the model for two simulated minutes and check it against its own description, not against a fly.