The Lampyridae are a family of insects in the beetle order Coleoptera. They are winged beetles, commonly called fireflies or lightning bugs for their conspicuous use of bioluminescence during twilight to attract mates or prey. Fireflies produce a "cold light", with no infrared or ultraviolet frequencies. This chemically produced light from the lower abdomen may be yellow, green, or pale red, with wavelengths from 510 to 670 nanometers.
About 2,000 species of fireflies are found in temperate and tropical climates. Many are in marshes or in wet, wooded areas where their larvae have abundant sources of food. Their larvae emit light and often are called "glowworms" in Eurasia and elsewhere. In the Americas, "glow worm" also refers to the related Phengodidae. In many species, both male and female fireflies have the ability to fly, but in some species, the females are flightless.
Fireflies tend to be brown and soft-bodied, often with the elytra, or front wings, more leathery than those of other beetles. Although the females of some species are similar in appearance to males, larviform females are found in many other firefly species. These females can often be distinguished from the larvae only because they have compound eyes. The most commonly known fireflies are nocturnal, although there are numerous species that are diurnal. Most diurnal species are not luminescent; however, some species that remain in shadowy areas may produce light.
A few days after mating, a female lays her fertilized eggs on or just below the surface of the ground. The eggs hatch three to four weeks later, and the larvae feed until the end of the summer. The larvae are commonly called glowworms (not to be confused with the distinct beetle family Phengodidae or the fly genus Arachnocampa.) Lampyrid larvae have simple eyes. The term glowworm is also used for both adults and larvae of species such as Lampyris noctiluca, the common European glowworm, in which only the nonflying adult females glow brightly and the flying males glow only weakly and intermittently.
Fireflies hibernate over winter during the larval stage, some species for several years. Some do this by burrowing underground, while others find places on or under the bark of trees. They emerge in the spring. After several weeks of feeding on other insects, snails and worms, they pupate for 1.0 to 2.5 weeks and emerge as adults. The larvae of most species are specialized predators and feed on other larvae, terrestrial snails, and slugs. Some are so specialized that they have grooved mandibles that deliver digestive fluids directly to their prey. Adult diet varies: some are predatory, while others feed on plant pollen or nectar. Some, like the European Glow-worm beetle, Lampyris noctiluca, have no mouth.
Most fireflies are quite distasteful to eat and sometimes poisonous to vertebrate predators. This is due at least in part to a group of steroid pyrones known as lucibufagins, which are similar to cardiotonic bufadienolides found in some poisonous toads.
Light production in fireflies is due to a type of chemical reaction called bioluminescence. This process occurs in specialized light-emitting organs, usually on a firefly's lower abdomen. The enzyme luciferase acts on the luciferin, in the presence of magnesium ions, ATP, and oxygen to produce light. Gene coding for these substances have been inserted into many different organisms (see Luciferase – Applications). Firefly luciferase is used in forensics, and the enzyme has medical uses — in particular, for detecting the presence of ATP or magnesium. All fireflies glow as larvae. Bioluminescence serves a different function in lampyrid larvae from the function served in adults. It appears to be a warning signal to predators, since many firefly larvae contain chemicals that are distasteful or toxic.
Light in adult beetles was originally thought to be used for similar warning purposes, but now its primary purpose is thought to be used in mate selection. Fireflies are a classic example of an organism that uses bioluminescence for sexual selection. They have a variety of ways to communicate with mates in courtships: steady glows, flashing, and the use of chemical signals unrelated to photic systems. The signal provides potential mates with information about the species of the signaller or its quality as a mate.
Some species, especially lightning bugs of the genera Photinus, Photuris, and Pyractomena, are distinguished by the unique courtship flash patterns emitted by flying males in search of females. In general, females of the Photinus genus do not fly, but do give a flash response to males of their own species.
Tropical fireflies, in particular, in Southeast Asia, routinely synchronise their flashes among large groups. This phenomenon is explained as phase synchronization and spontaneous order. At night along river banks in the Malaysian jungles, fireflies synchronize their light emissions precisely. Current hypotheses about the causes of this behavior involve diet, social interaction, and altitude. In the Philippines, thousands of fireflies can be seen all year-round in the town of Donsol (called aninipot or totonbalagon in Bicol). In the United States, one of the most famous sightings of fireflies blinking in unison occurs annually near Elkmont, Tennessee, in the Great Smoky Mountains during the first weeks of June. Congaree National Park in South Carolina is another host to this phenomenon.
Female Photuris fireflies are known for mimicking the mating flashes of other "lightning bugs" for the sole purpose of predation. Target males are attracted to what appears to be a suitable mate, and are then eaten. For this reason, sometimes, Photuris species are referred to as "femme fatale fireflies."
Many fireflies do not produce light. Usually these species are diurnal, or day-flying, such as those in the genus Ellychnia. A few diurnal fireflies that inhabit primarily shadowy places, such as beneath tall plants or trees, are luminescent. One such genus is Lucidota. Non-bioluminescent fireflies use pheromones to signal mates. This is supported by the fact that some basal groups do not show bioluminescence and, rather, use chemical signaling. Phosphaenus hemipterus has photic organs, yet is a diurnal firefly and displays large antennae and small eyes. These traits strongly suggest pheromones are used for sexual selection, while photic organs are used for warning signals. In controlled experiments, males coming from downwind arrived at females first, indicating males travel upwind along a pheromone plume. Males were also found to be able to find females without the use of visual cues, when the sides of test Petri dishes were covered with black tape. This and the facts that females do not light up at night and males are diurnal point to the conclusion that sexual communication in P. hemipterus is entirely based on pheromones.
Firefly systematics, as with many insects, are in a constant state of flux, as new species continue to be discovered. The five subfamilies listed above are the most commonly accepted ones, though others, such as the Amydetinae and Psilocladinae, have been proposed. This was mainly done in an attempt to revise the Lampyrinae, which bit by bit had become something of a "wastebin taxon" to hold incertae sedis species and genera of fireflies. Other changes have been proposed, such as merging the Ototretinae into the Luciolinae, but the arrangement used here appears to be the most frequently seen and stable layout for the time being. Though most groups appear to be monophyletic, some (e.g., the tribe Photinini) are perhaps better divided.
Two groups of subfamilies seem to exist: one containing many American and some Eurasian species in the Lampyrinae and Photurinae, and one, predominantly Asian, made up from the other subfamilies. While the subfamilies as understood here are, in general, monophyletic, a few genera still need to be moved for the subfamilies to accurately represent the evolutionary relationships among the fireflies.
The Rhagophthalmidae are a glow worm-like lineage of Elateroidea. They have in the recent past usually been considered a distinct family, but whether this is correct is still disputed. Indeed, they might be the only close relative of the puzzling firefly genus Pterotus, which sometimes is placed in a monotypic subfamily.
The genus Phausis, usually placed in the tribe Photinini of the Lampyrinae, might represent another rather distinct lineage instead.