Common Eastern Firefly (Photinus pyralis): Facts and Flashes

What does the common eastern firefly look like?

The common eastern firefly is a slender, flattened beetle usually 10–14 mm (0.4–0.55 in) long, with extremes of about 9–19 mm (0.35–0.75 in). It is one of the larger Photinus fireflies in its range. Its wing covers, or elytra, are dark brown to blackish with thin yellow edges, and they are softer and more leathery than those of most beetles.

The head is hidden from above under a shield-like plate called the pronotum. In this species the pronotum is pale yellow with a black central spot surrounded by pink or reddish patches, a useful field mark. The antennae are long and thread-like.

The light organs, or lanterns, are on the underside of the abdomen near the tip. In males the pale, creamy-yellow lantern covers most of the last few abdominal segments; in females it is smaller and limited to a single segment, so the sexes can often be told apart by turning them over.

The species was named by Carl Linnaeus in 1767. Photinus comes from a Greek word meaning shining, and pyralis from pyr, the Greek word for fire. In North America it is known as the common eastern firefly, the big dipper firefly or simply the lightning bug.

Is a firefly a beetle?

Yes. Fireflies belong to the beetle order Coleoptera, the largest group of animals on Earth, described in how many beetle species there are. They form the family Lampyridae, with roughly 2,000 to 2,400 described species worldwide and about 170 in the United States and Canada.

Not all fireflies flash. Some day-active species have no light as adults and find mates by scent, while in others, such as the European glow-worm, wingless females glow steadily. The common eastern firefly belongs to the flashing “lightning bug” group in which both sexes signal with light. When its genome was sequenced in 2018, scientists found evidence that fireflies and glowing click beetles evolved their light separately.

Habitat and range

The common eastern firefly lives throughout much of the United States east of the Rocky Mountains. It is the most widespread and abundant firefly in many eastern and midwestern states and one of the few that thrives near people.

It needs damp ground for its eggs and larvae, and it favors open, grassy places: lawns, meadows, old fields, parks, golf course roughs, woodland edges and the banks of streams and ponds. It is common in suburbs and city parks where grass is not cut too short and night lighting is limited.

Adults are on the wing from late spring into summer, appearing earlier in the warm South than in the North, and most displays in the eastern states take place in June and July. Unlike many fireflies that are active only in full darkness, this species begins flashing at twilight, which is why it is the lightning bug that most people see in their backyards.

Common eastern firefly and look-alikes

Several other fireflies share its range, and the most frequent source of confusion is the genus Photuris. Photuris fireflies are usually larger, with longer legs and a more elongated body, and many have pale stripes on their wing covers. They tend to start flashing later in the evening, often higher in trees and shrubs, and their flash patterns differ from the J-shaped flash of Photinus pyralis.

Other Photinus species look very similar, and many can be told apart reliably only by their flash patterns or by close examination. A firefly that flashes low over a lawn at dusk, drawing a rising “J” about every five or six seconds, is very likely to be a common eastern firefly.

Europe has no Photinus fireflies at all. Its best-known species, the common glow-worm, signals in a completely different way, as described in are there fireflies in Europe.

The J-shaped flash: how fireflies court

Courtship begins around sunset. Males take off and fly slowly and fairly low over the grass, flashing as they go. Each flash lasts about a third to half a second, and a patrolling male typically flashes about every 5 to 7 seconds, faster on warm nights and slower on cool ones. During each flash he dips and then climbs, so the light draws a “J” or a scoop shape in the air.

Females perch on grass stems and low plants. When a female sees the flash of a male of her own species, she answers with a single flash of her own, usually about 2 seconds after his. That precise delay is the key to the code. The male turns toward her, flashes again, and the dialogue continues until he lands beside her.

Several males often home in on the same female. During mating the male passes a nuptial gift, a package of sperm and nutrients that helps the female produce eggs, and females may mate with different males on different nights. Read more about the chemistry and meaning of the signals in why fireflies glow.

How fireflies make light

Firefly light is a form of bioluminescence, light produced by a chemical reaction inside a living animal. In the lantern, an enzyme called luciferase combines a compound called luciferin with oxygen and the cell’s energy molecule, ATP, with magnesium as a helper. The reaction gives off a yellow-green glow and very little heat, so firefly light is often called cold light.

A firefly controls its flashes by controlling the supply of oxygen to the light-producing cells, which receive air through a fine network of breathing tubes. A layer of reflective cells behind the lantern bounces light outward. The light of this species spans roughly 520 to 620 nanometers, from green to orange-yellow, and appears yellow-green to our eyes. Fireflies are just one of many glowing creatures; see the list of bioluminescent animals.

Life cycle: from egg to adult

After mating, a female lays her eggs on or just below the surface of damp soil. The eggs themselves give off a faint glow. They hatch after about three to four weeks into flattened, segmented larvae that live in soil, moss and leaf litter.

The larvae are the main feeding stage. They hunt earthworms, snails, slugs and other soft-bodied invertebrates, injecting digestive fluids that paralyze and liquefy the prey. Larvae glow faintly, a warning to predators that they taste bad. They spend the winter underground and usually take one to two years to grow.

When fully grown, a larva builds a small chamber in the soil and pupates. About one to two weeks later the adult emerges, in late spring or early summer. Adults eat little or nothing and live only a few weeks, surviving on energy stored by the larva.

Defenses and predators

Common eastern fireflies contain lucibufagins, bitter steroids that make them distasteful or toxic to many birds, lizards, spiders and other predators. When grabbed, they can ooze drops of sticky, foul-tasting blood from their wing covers, a defense known as reflex bleeding.

Their most famous enemies are other fireflies. Females of the genus Photuris, nicknamed “femmes fatales”, copy the answering flashes of Photinus females, lure in the males and eat them. In doing so, they take up the lucibufagins they cannot make themselves and use them for their own defense.

Fireflies in science

The common eastern firefly has had a large impact on biology. Its luciferase gene was cloned in 1985, and in 1986 scientists inserted it into tobacco plants, which glowed when supplied with luciferin. Firefly luciferase is now one of the most widely used tools in laboratories, for example to detect ATP from living cells and to show when genes are switched on. Most luciferase used today is produced from cloned genes rather than collected from wild fireflies.

In 2018 an international team published the genome of the common eastern firefly alongside those of a Japanese firefly and a glowing Caribbean click beetle. The comparison supported the idea that fireflies and click beetles evolved light production independently, each from enzymes related to ones that ordinary insects use to process fatty acids. The genome also gave clues to how fireflies make their defensive lucibufagins.

Conservation status and threats

The common eastern firefly is listed as Least Concern on the IUCN Red List (assessed in 2021). It is widespread, common and adaptable, unlike many firefly species with small ranges or specialized habitats.

Even so, it faces the same pressures as other fireflies: loss of damp grassland and woodland edges, pesticides that kill larvae and their prey, and light pollution. In field experiments published in 2016, artificial lighting reduced courtship and mating success in this species. The wider picture is explained in are fireflies disappearing.

How to see common eastern fireflies

The best time to look is in June and July, from about sunset until full darkness, on warm, humid, windless evenings. Lawns next to woodland, damp meadows and stream edges are good places. Turning off outdoor lights, leaving some areas of grass uncut and watching quietly rather than collecting the insects all help fireflies in a backyard.