Firefly

A firefly’s light is almost perfectly efficient. Close to 100 percent of the energy it uses comes out as light, with virtually none wasted as heat, which is why you can hold a glowing firefly in your palm and feel nothing. Fireflies are a family of light-producing beetles, the Lampyridae, found on every continent except Antarctica.

An old incandescent bulb managed about 10 percent, wasting the rest as heat. A good modern LED reaches somewhere in the region of 50 percent.

Nothing humans have built matches the beetle.

Fireflies are the family Lampyridae, with more than 2,000 species, so there is no single scientific name. The common eastern firefly of North America is Photinus pyralis.

• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Coleoptera
• Family: Lampyridae

That order is the point everyone misses: Coleoptera is the beetles. A firefly is not a fly, and a lightning bug is not a bug in the strict sense either. It is a soft-bodied beetle, and the correct name for the family is lampyrid beetles.

Glow-worms are the same family. The name usually refers to the larvae, which glow, or to adult females of species in which the female is wingless and larva-like. A glow-worm is normally a female or young firefly rather than a different animal.

The light comes from a chemical reaction in an organ in the abdomen called the lantern, and the chemistry is well understood:

• Luciferin is the fuel, a compound that releases light when it is oxidised.
• Luciferase is the enzyme that drives the reaction.
• Oxygen is the trigger, delivered through tubes called tracheoles, and this is how the firefly switches the light on and off: it controls the oxygen supply, not the chemicals.
• A reflective layer behind the lantern bounces the light outward, so very little is lost inward.

The switching is faster than oxygen diffusion alone should allow, and the explanation found in the early 2000s was nitric oxide, which the firefly produces to let oxygen flood through to the light-producing cells in a controlled pulse.

Luciferase has become one of the most useful tools in biology. Because the reaction needs ATP, the molecule that carries energy in living cells, luciferase can be used to measure whether cells are alive and active, and the gene is spliced into other organisms as a reporter so that researchers can literally watch a gene switch on as a glow. Firefly luciferase is used in cancer research, drug screening, food safety testing and detecting bacterial contamination.

For decades this created a problem: luciferase was harvested from wild fireflies, and tens of thousands were collected. It is now produced synthetically.

Fireflies are 0.2 to 1 inch long and weigh only a few hundredths of a gram, with large species heavier but still a fraction of an ounce.

They are soft-bodied, unlike most beetles, usually brown or black with yellow, orange or red markings, and the lantern is on the underside of the last abdominal segments, visible as a pale patch when the insect is not glowing.

Males usually have large eyes, since they are looking for a female’s answering flash, and in many species females have small wings or none at all and never fly.

The light color varies by species, from green through yellow to orange, and species active at dusk tend toward yellow, which stands out better against green foliage in fading light, while fully nocturnal species glow greener.

Fireflies live on every continent except Antarctica, most abundantly in warm, humid places, and the greatest variety is in tropical Asia and the Americas.

What they need is damp. Firefly larvae live in soil, leaf litter, rotting wood, under bark or at the margins of marshes and streams, and a few species have aquatic larvae with gills. Dry, tidy, well-drained ground supports no fireflies.

This is why fireflies vanish from manicured landscapes. The adults are the visible stage, but the larvae spend one to two years underground in moist litter, and clearing leaves, draining ground and treating lawns removes the part of the life cycle nobody sees.

Not all species flash. Many, particularly in western North America, are active by day and have no light display at all as adults, which is why much of the western United States has fireflies but no visible firefly season.

The larvae are unambiguous predators, eating snails, slugs, worms and other soft-bodied invertebrates. They inject a paralyzing digestive fluid and consume the liquefied prey, and a firefly larva will track a snail’s slime trail to find it.

The adults are variable, which is why the family reads as omnivorous overall. Some eat nectar or pollen, many eat nothing at all in their short adult lives, and a few are predators.

The predatory adults are the interesting case, and the female Photuris fireflies are the reason. They are known as femmes fatales, and what they do is mimic the answering flash of females of other firefly species. A male of that other species flies down expecting to mate, and she eats him.

She is not only after a meal. By eating those males she acquires defensive chemicals called lucibufagins that her own species cannot make, which make her and her eggs poisonous to spiders and birds. The deception is a chemical supply chain.

The flashing is courtship, and it is a species-specific code. Each species has its own pattern: the number of flashes, their length, the interval between them, the color, and the flight path during the flash. Photinus pyralis males fly in a rising J-shape while flashing, which is why the species is called the big dipper firefly.

The male flashes his pattern; a female on the ground or in vegetation answers with a flash at a precise delay. Get the delay wrong and nothing happens. That precision is what makes the Photuris mimicry possible, and it is also how dozens of species can share a single meadow without confusion.

Then there is synchrony. In parts of Southeast Asia, whole riverbank trees full of fireflies flash in unison, thousands of insects pulsing together for hours, and in the Great Smoky Mountains Photinus carolinus does the same, drawing enough visitors that access is now run by lottery. How it works is a matter of each insect adjusting its own rhythm slightly in response to its neighbors, with no leader and no signal from outside, and it has become a standard example in the mathematics of coupled oscillators.

The larvae glow too, including species whose adults do not, and in their case the light is a warning that they taste foul.

The life cycle is heavily weighted toward the stage nobody sees.

After mating, the female lays 50 to 100 eggs in damp soil or moss, and in some species the eggs themselves faintly glow. They hatch in three to four weeks.

The larva then lives for one to two years, hunting snails underground and through leaf litter, molting repeatedly, and overwintering, sometimes burrowed into soil or under bark.

It pupates for one to three weeks and emerges as an adult, which lives two to four weeks and in many species does not eat at all. The adult exists to breed.

So a firefly spends almost its whole life as a predatory grub in the dirt, and the glowing insect in the summer air is the last few weeks of it.

Spiders, frogs, toads, birds, bats and predatory insects eat fireflies, though many are chemically defended and the flash itself may double as a warning. Fireflies are notably toxic to some reptiles, and bearded dragons have died from eating a single one, which matters to anyone keeping them.

Firefly numbers are falling in many places, and the causes are specific:

• Light pollution. This is the one unique to them. Artificial light drowns out the flashes, so males and females cannot find each other. Studies have shown reduced mating activity under streetlights and floodlights, and in bright areas the display simply fails.
• Habitat loss. Drainage of wetlands and marsh margins, and the loss of damp leaf litter to development and tidy gardening.
• Pesticides. Lawn treatments kill larvae in the soil, and broad-spectrum insecticides remove the snails they eat.
• Over-collection at famous sites, and historically for luciferase.

Most firefly species have never been assessed, which is the honest answer. Of more than 2,000 species, only a fraction have been evaluated, and of those assessed a meaningful share are threatened.

The Bethany Beach firefly in Delaware is Endangered and confined to a handful of coastal wetlands. Several Malaysian and Japanese species have declined with the loss of mangrove and rice-paddy habitat.

It is also worth being careful about the widely repeated claim that fireflies are vanishing everywhere. The declines are real and well documented in specific places, but there is no global firefly census, and in much of the world nobody is counting. The uncertainty is itself part of the problem.

What helps is unusually easy, which makes this one of the most actionable conservation cases on the site: turn off outdoor lights at night, leave leaf litter where it falls, let a corner stay damp and long, and skip the lawn pesticide.

Firefly larvae are significant predators of snails and slugs, spending a year or two working through the soil fauna, and they are prey themselves at both stages.

As indicators they are excellent, because they need moisture, intact litter, low pesticide use and darkness all at once. A garden with fireflies in it is a garden that is working.

Their contribution to science is out of all proportion to their size. Luciferase from this beetle is used daily in laboratories worldwide to track gene activity, test drugs and detect contamination, and the firefly’s efficiency has been studied directly by engineers working on LED design, with one project improving light extraction by copying the microscopic scale structure on a firefly’s lantern.

And they are worth protecting for a reason that has nothing to do with any of that. A field full of synchronized fireflies is one of the few genuinely astonishing things a person can see without traveling anywhere special, and it is being switched off by porch lights.