Invertebrates
Firefly
Photinus pyralis · also called lightning bug, common eastern firefly, big dipper firefly
The firefly is a soft-bodied beetle that turned chemistry into a language. On warm summer nights, the males rise from the grass flashing coded messages, the females flash back, and an entire courtship is conducted in points of cold green light.
Overview
Fireflies — also called lightning bugs — are beetles in the family Lampyridae, with over 2,000 species worldwide. Photinus pyralis, the “big dipper” firefly, is the common backyard species of the eastern and central United States. Fireflies favour damp, undisturbed ground near water — wet meadows, marsh edges, streambanks, and old lawns and woodland margins — because their larvae live in moist soil and leaf litter. Adults appear for just a few weeks in late spring and early summer.
Cold light
A firefly’s glow comes from a bioluminescent reaction in a light organ under the last segments of the abdomen. The compound luciferin is oxidised in a reaction catalysed by the enzyme luciferase and powered by ATP, and almost all the released energy comes out as light — making it one of the most efficient light sources known, in contrast to an incandescent bulb, which loses most of its energy as heat. The firefly turns the flash on and off by controlling how much oxygen reaches the reaction.
Talking in flashes
Firefly light is primarily a mating signal, and each species has a distinct code — a particular colour, flash length, number of flashes, and the interval between them. In the common eastern firefly, a flying male produces a rising “J”-shaped flash roughly every six seconds; a female of the same species, waiting in low vegetation, answers with a single flash after a set pause, and the exchange guides the male to her.
Femme fatale fireflies
Not every answering flash is an invitation. Females of the genus Photuris mimic the response signals of other firefly genera. A male that flies down to what he takes to be a mate is grabbed and eaten — and the predatory female also acquires defensive toxins from her victims that she cannot synthesise herself, using them to protect herself and her eggs from spiders and birds.
Life cycle
Most of a firefly’s life is spent as a larva — a flattened, segmented, glowing “glow-worm” that hunts snails, slugs, and earthworms in the soil, injecting them with digestive enzymes. After one or two years the larva pupates, and the adult emerges to spend its brief life flashing and mating. Many adult fireflies do not feed at all.
Status and decline
The common eastern firefly is still widespread, but firefly populations are declining across much of the world, and several species are now formally threatened. The three main drivers are light pollution (which jams the flash signals), habitat loss (draining and developing the wet ground larvae need), and pesticides. Turning off outdoor lights in summer, leaving areas of long grass and leaf litter, and avoiding lawn chemicals all help local fireflies persist.
Firefly: common questions
How do fireflies make light?
In a light organ near the tip of the abdomen, a molecule called luciferin reacts with oxygen, driven by the enzyme luciferase and the cell's energy currency ATP, and releases the energy almost entirely as light rather than heat — a "cold light" that is close to 100% efficient. The firefly controls the flash by regulating the oxygen supply to the light organ, switching it on and off in patterns. The light is usually yellow-green, the wavelength that carries best through the dusk.
What are fireflies saying when they flash?
Mostly, males are advertising and females are answering. Each species has its own signature — a specific flash colour, duration, number of pulses, and interval. A cruising male of the common eastern firefly makes a distinctive upward-swooping flash every five or six seconds; a receptive female of his species, perched in the grass, replies with a single flash after a precise delay, and he homes in on her. Get the timing or pattern wrong and there is no response.
Do some fireflies eat other fireflies?
Yes. Females of the genus Photuris are predators of other fireflies. A Photuris female watches for the flash of a male of a different genus, such as Photinus, then imitates that species' female response code. The deceived male lands expecting to mate and is seized and eaten. The predatory female also gains defensive chemicals (lucibufagins) from her prey, which she cannot make herself, and passes them to her eggs.
Why are fireflies disappearing?
Several linked causes. Light pollution from streetlights, buildings, and car headlights drowns out the faint flash signals so males and females cannot find each other. Habitat loss — the draining of wet meadows and marshes, and the paving and manicuring of the damp, leaf-littered ground the larvae need — removes breeding sites. Pesticides kill both larvae and their snail and slug prey. Because many firefly species are localised and short-lived as adults, these pressures add up quickly.