In your yard: fireflies


Flash, flash, flash! In May, June, and July, their nighttime displays are fascinating to kids and adults, alike. Have you ever wondered about fireflies and how they do that? Would you be surprised to learn that fireflies are not flies at all? They’re beetles!

The fireflies, also called lightning bugs, you see flashing at night are one or more of about 2,000 species of beetles with this amazing ability. They inhabit all continents, except Antarctica. In North America, there are more than 150 species. Their wingless larvae are sometimes called glowworms. Fireflies are the state insect of Tennessee and Pennsylvania.

Firefly standing a person's finger

Firefly, Pleotomus sp. (Alejandro Santillana / Flickr; cc by 2.0)

Fireflies like fields and forest edges near rivers, lakes, streams, marshes, ponds and other wet areas. They’re greatest numbers are found from Kansas eastward, where those environments are widely available. In the more-arid West, they inhabit only moist areas. Fireflies are nocturnal (a few are diurnal), so that’s why we see them as night comes on.

Fireflies are in the order Coleoptera and the family Lampyridae. Fossil evidence shows they date back at least 26 million years, to the Paleocene Epoch. But, fireflies today are disappearing. No one knows precisely why, but it’s thought that human development is the cause: light pollution, which interrupts firefly flash patterns; destruction of forests; and pesticides, which kill both fireflies and their prey.

Firefly in flight, Photinus pyralis. (Terry Priest / Flickr; cc by 2.0)

Fireflies range in size from 3/8 to 1 inch long (5 mm to 25 mm), with most being about an inch. They have a soft body and are generally brown. Like all other insects, they have three body parts: head, thorax, and abdomen. 

There are three pairs of legs attached to the thorax. They have two pairs of wings. The forewings are hardened and look a bit leathery. Called elytra (ELLA-truh), they protect the hindwings, which are delicate and used for flight. The elytra cover the hindwings when they’re at rest and are moved out of the way for flight, as you can see in the photo above.

There’s nothing very remarkable-looking about fireflies in the daytime, but at night you can’t miss their most notable physical characteristic — the seemingly magical flashing that comes from their abdomen. (A few species light up in the daytime, too.) In some species, only one sex can produce a light. 

The firefly’s light is the result of a chemical reaction called bioluminescence, which occurs basically like this: First, the firefly injects oxygen into light-emitting organs (called photocites) located on the underside of its abdomen. This causes a pigment called luciferin,** which glows under certain conditions, to be catalyzed by an enzyme called luciferase. Then, Voilà! A light turns on! Calcium and adenosine triphosphate also play a role in the process. 

Firefly light may be yellow, green or orange, depending on the species. The light is referred to as “cold light,” because it has no infrared or ultraviolet component, which would generate heat. Read more in depth about this here.

Fireflies can start and stop reactions by controlling oxygen into their photocites. Oxygen in: light on. Oxygen not in: light off. For more about how fireflies breathe, click to All about beetles

Fireflies make their light with higher efficiency than any other bioluminescent creature. They’ve been said to have nearly 100 percent light production efficiency, but more recent measurements have shown it to be about 40 percent. This means that four photons of light get emitted for every 10 chemically transformed luciferin molecules. That may not seem like a lot, but, by comparison, a standard incandescent lightbulb shines with only 10 percent efficiency.

Life cycle
Fireflies undergo complete metamorphosis, which has four stages: egg, larva, pupa, and adult.

Firefly larva. (Katja Schulz / Flickr; cc by 2.0)

Many firefly species have unique flash patterns and use them to identify other members of their kind, as well as to attract the opposite sex. Flashing patterns may be as simple as a single flash up to a series of pulsing flashes, or even a continuous glow. With some species, both sexes flash, in others only the male does. Some species don’t flash at all; instead, they use pheromones to attract mates. There are still others that use both flashing and pheromones.

Synchronous firefly (Photinus carolinus) males are multi-talented; they may flash randomly, synchronize their flashes or even flash in waves across the hillsides in the Great Smoky Mountains where they live. There’s an interesting video about them here.

To find mates, males fly around flashing, while females perch on vegetation and observe them. Females choose mates based on species-specific flash patterns (and/or pheromones, with some species.) When a female sees a male that interests her, she flashes back, and a relationship ensues. A few days after mating, the female lays about 500 eggs (singly or in groups) in or on damp soil.

Eggs hatch in about a month. In some species, the eggs glow and will flash when gently tapped. The larvae live in rotting wood, forest litter and under bark, feeding on worms, snails, and insects. Ahead of winter they burrow underground or move in or under tree bark to hibernate. Some species hibernate for several years.

Larvae emerge in the spring and feed for several weeks, then pupate for one to three weeks. Larvae of some species live in water and feed on aquatic snails until it’s time to move onto land. 

Adults feed on pollen and nectar, although some don’t eat at all.  They live three to four weeks. 

A firefly’s flashing light is a warning to predators not to bite. They aren’t tasty, and animals soon learn not to attack them. When bitten, fireflies bleed through a process called “reflex bleeding.” Their blood contains chemicals that are bitter-tasting to most animals and even poisonous to some. (Note: Fireflies should never be fed to pet reptiles.) They also use their lights to defend their territory from other firefly species.

Fireflies love warmth and moisture. Those of us living in suitable habitat, get to see fireflies in our yards and gardens. Most, though, live in fields, forests, and marshes near rivers, lakes, streams, and ponds. In fact, some firefly larvae are aquatic.

You may be tricked into thinking a bug sporting a light is a firefly, but not all glowing insects are. “Click beetles” in the genus Pyrophorus have two glowing lights on their head, two on the back of the head and one under the abdomen. Railroad worms (the larvae of beetles in the genus Phrixothrix) have green lights lined along their body and red lights on their head. Click beetles and railroad worms don’t flash, but the Firefly-mimicking Longhorn Beetle (Mecas rotundicollis) almost exactly mimics the abdominal flashing of a firefly.

Catching fireflies
It’s best to use a net to gently entrap them. If using only your hand, don’t squeeze, they’re fragile. If you want to watch them for a while, carefully put them in a jar that holds a moist paper towel. Puncture the lid to let in air. Watch them for only a day or two and then let them go at night. It’s important to let them go unharmed; our planet is losing them.

*Top photo: Fireflies (Xenmate / Flickr; cc by 2.0)
**From Latin lucifer, meaning “light-bearer”

More reading:

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