Is Firefly Night free?

Yes. It runs in your browser on phones, tablets and computers, with no download and no account.

Do fireflies really flash in sync?

Some species do. Photinus carolinus in the Great Smoky Mountains and Pteroptyx fireflies in Southeast Asian mangroves flash in unison. Each firefly nudges its timing when it sees neighbours flash, so the group falls into step without a leader.

Why does a firefly glow?

An enzyme, luciferase, uses ATP and oxygen to oxidise luciferin, and the reaction gives off light instead of heat. Fireflies use the flashes to find mates.

Why can I only catch them when they glow?

At night you can only see a firefly when it lights up, so that is the moment to catch it. It also makes timing the whole game.

Is it OK to catch real fireflies?

Gently, briefly, and let them go the same night. Firefly numbers are falling in many places because of habitat loss and light pollution, so turning off outdoor lights helps them most.

Firefly Night

Catch them while they glow. Watch them learn to blink together. Back to the game

How to play

It is a warm summer evening in a meadow drawn on notebook paper. Fireflies drift over the grass, and each one blinks on its own little clock. Tap or click a firefly at the moment it glows and it floats gently into your paper jar. Tapping a dark one does nothing: you have to wait for its light. When the jar is full, the fireflies inside keep blinking and slowly nudge each other into step. Once every one of them flashes at exactly the same moment, the jar glows warmly and you can release them in a shower of light. That finishes the hour, and the night gets darker.

Press Flash my light to blink a lantern of your own. Fireflies that were nearly ready to flash answer you straight away, so a well-timed flash makes a whole group light up together (handy for catching), and flashing in rhythm with the jar helps it find the beat. Later hours bring more fireflies, shorter glows, a hungry frog (tap it to sing it to sleep) and moths that look like fireflies but never flash. Catch a moth by mistake and the jar gets jostled out of step. If the moon sets before you release a jar, the night is over. Your score is the number of fireflies you released in sync, and your best night is saved.

Keyboard: the arrow keys move your net, Space or Enter catches the glowing firefly closest to it, F flashes your light and R releases a glowing jar.

Cold light: how a firefly glows

The light comes from a chemical reaction in the firefly's lantern, the pale segments at the end of its abdomen. An enzyme called luciferase uses the cell's energy molecule ATP and oxygen to oxidise a small molecule called luciferin. The product is left in an excited state and gives up its energy as a photon. The reaction is remarkably efficient: a large share of the reactions make light (quantum yield measurements range from about 40% to over 90%), and almost no energy is lost as heat, which is why it is called "cold light". An old incandescent bulb turns only about 2% of its power into visible light. Firefly light peaks near 560 nanometres, a yellow-green close to where human eyes are most sensitive. The firefly switches the flash on and off by controlling the oxygen that reaches the lantern, using nitric oxide as a signal.

Why they fall into step

Each firefly in the game is a pulse-coupled oscillator: an internal clock that charges up, flashes, and resets. When a firefly sees a neighbour flash, its own clock jumps forward a little, more so the closer it already was to flashing. In 1990 the mathematicians Renato Mirollo and Steven Strogatz proved that a group of identical oscillators coupled this way almost always ends up perfectly synchronised, whatever the starting times: whenever two clocks are close, one flash pulls the other over the edge, and once they fire together they stay together. Real fireflies are not identical, so some species, such as Pteroptyx malaccae, also nudge the speed of their own clock toward the group's rhythm (Bard Ermentrout modelled this in 1991). The game's fireflies do both. Nobody conducts the orchestra. The same mathematics, studied more broadly in the Kuramoto model, describes heart pacemaker cells, crickets chirping and applause turning into rhythmic clapping.

Real synchronous fireflies do this in the wild. Photinus carolinus in the Great Smoky Mountains, around Elkmont in Tennessee, put on a display for about two weeks each June, with males flashing in bursts of five to eight and then going dark together for several seconds. In Southeast Asia, Pteroptyx fireflies gather in mangrove trees along rivers and blink in unison all night. Most other species use flashes as a private courtship code: a male flies and flashes a pattern, and a female of the same species answers from the grass after a precise delay. Some predatory Photuris females even copy other species' answers to lure males in.

Tips

Watch one firefly for a couple of blinks to learn its rhythm, then tap just as it lights up.
Flash your light near a group: the ones that answer all glow at once.
Flash in time with the jar's blinks to pull stragglers into step.
Moths have a steady, pale shimmer. Fireflies pulse.

Love emergent order? Watch a flock form in ../murmuration/ or follow a scent trail in ../follow-the-ants/.

Questions people ask

Is Firefly Night free?: Yes. It runs in your browser on phones, tablets and computers, with no download and no account.
Do fireflies really flash in sync?: Some species do. Photinus carolinus in the Great Smoky Mountains and Pteroptyx fireflies in Southeast Asian mangroves flash in unison. Each firefly nudges its timing when it sees neighbours flash, so the group falls into step without a leader.
Why does a firefly glow?: An enzyme, luciferase, uses ATP and oxygen to oxidise luciferin, and the reaction gives off light instead of heat. Fireflies use the flashes to find mates.
Why can I only catch them when they glow?: At night you can only see a firefly when it lights up, so that is the moment to catch it. It also makes timing the whole game.
Is it OK to catch real fireflies?: Gently, briefly, and let them go the same night. Firefly numbers are falling in many places because of habitat loss and light pollution, so turning off outdoor lights helps them most.