Why Do Fireflies Glow? Bioluminescence and Flash Signals

Fireflies glow to find a mate

For most glowing fireflies, light is a language of courtship. In flashing “lightning bugs” of the Americas, males patrol at dusk, blinking a pattern that belongs to their own species. Females wait on grass and shrubs and reply with flashes of their own.

The common eastern firefly of North America is a good example. A male gives a short flash, lasting about a third to half a second, roughly every 5 to 7 seconds while dipping and rising in flight. A female of the same species answers about 2 seconds later. The male then flies toward her, and the exchange is repeated until he finds her.

Species differ in how long each flash lasts, how many pulses it contains, the gap between flashes, the color of the light, the height at which males fly and the shape of the flight path. Temperature changes the timing: on warm nights flashes come faster. A female responds only when the pattern and her own delay match those of her species, which keeps related fireflies from mixing.

Not all fireflies flash. In the European glow-worm, the wingless female glows steadily from a grass stem while flying males search for her light, as described in are there fireflies in Europe. Some day-active fireflies make no light as adults and find mates by scent instead.

Firefly light is also a warning

Fireflies contain lucibufagins, bitter steroids related to the toxins in some toads. Many birds, lizards and spiders learn to avoid them. A glow is a reliable way to advertise this defense at night, a strategy called aposematism, or warning coloration.

As far as is known, the larvae of all fireflies glow, even in species whose adults have no light. Comparisons of firefly family trees suggest that light first evolved as a larval warning signal and only later became a courtship code in adults. In some species the eggs and pupae glow faintly too.

How does a firefly make light?

Firefly light is bioluminescence, light made by a chemical reaction in a living organism. Inside the lantern, near the tip of the abdomen, are light-producing cells called photocytes. They contain two key chemicals:

• Luciferin, the compound that gives off light.
• Luciferase, the enzyme that drives the reaction.

When luciferase combines luciferin with oxygen and ATP, the energy-carrying molecule of cells, with magnesium as a helper, the luciferin is changed into an excited form that releases its energy as a particle of light. Behind the light cells, a layer of reflective cells packed with tiny crystals acts like a mirror and directs the light outward.

How do fireflies switch their light on and off?

Oxygen is the switch. A firefly’s lantern is threaded with fine air tubes called tracheae, and light is produced only when oxygen reaches the photocytes. Nerve signals trigger each flash. A 2001 study proposed that these signals release nitric oxide, a gas that briefly lets oxygen flow to the light cells, producing a flash; this remains one of the leading explanations.

Why is firefly light called cold light?

An old-fashioned light bulb turns most of its energy into heat. A firefly’s reaction releases very little heat, so its lantern stays cool. Measurements published in 2008 found that about 41 percent of reacting luciferin molecules give off a particle of light, a high efficiency for a chemical light source.

Firefly colors range from green through yellow to orange, depending on the species and the exact structure of its luciferase. The common eastern firefly glows yellow-green. Fireflies are among many glowing creatures, from jellyfish to deep-sea fish; see the list of bioluminescent animals, or how a deep-sea predator uses light as bait in why anglerfish have a light.

Synchronous fireflies

In a few species, many males flash in unison. North America’s best-known example is Photinus carolinus in the Great Smoky Mountains, where displays near Elkmont, Tennessee, peak in early June. Each male gives four to eight bright flashes over two to four seconds, then goes dark for eight to twelve seconds, and thousands fall into step. Huge synchronized displays also occur in mangroves in Southeast Asia.

Why they synchronize is still debated. One leading idea is that a shared dark pause helps males spot the dimmer answering flashes of females without being dazzled by rivals.

Firefly “femmes fatales”

Light signals can also be used to deceive. Females of the North American genus Photuris copy the answering flash of Photinus females. When a Photinus male flies down expecting a mate, the Photuris female catches and eats him. Studies have shown that she gains more than a meal: she takes up his lucibufagins, which her own body cannot make, and becomes better protected against spiders and other predators.

So firefly light does several jobs at once: it helps mates find each other, warns predators away and, in a few species, lures prey.