Why and how fireflies light up

Yellow glowing dotted lines above grass in nighttime scene.
Fireflies via Matt Pollock in upstate New York.

Cold light

Lightning bugs and fireflies are one and the same insect, a kind of beetle. You’re most likely to see them in warm weather, when rainfall has been plentiful. Many a child has spent a summer evening chasing fireflies. And maybe you’ve wondered, why do fireflies light up? And how do they do it?

Let’s talk about the how first. The light of a firefly is a chemical reaction caused by an organic compound in their abdomens. The compound is called luciferin. As air rushes into a firefly’s abdomen, it reacts with the luciferin. It causes a chemical reaction that gives off the firefly’s familiar glow. This light is sometimes called cold light because it generates so little heat.

According to

Scientifically, fireflies are classified under Lampyridae, a family of insects within the beetle order Coleoptera, or winged beetles. There are estimated to be 2000+ firefly species spread across temperate and tropical zones all over the world.

Fireflies light up for safety

The firefly can regulate the airflow into the abdomen to create a pulsating pattern.

And that brings us to the why.

Some experts think the firefly’s flashy style may warn predators of the insect’s bitter taste. On the other hand, some frogs don’t seem to mind the flavor. They eat so many fireflies that they themselves begin to glow.

And romance

Male fireflies also light up to signal their desire for mates, and willing females attract the males with flashes of their own.

But not all the flashing of fireflies is motivated by romance. While each firefly species has its own pattern of flashing, some females imitate the patterns of other species. Males land next to them, only to be eaten alive.

So the next time you see a firefly, keep in mind that its flickering isn’t just a wonder of the night. It’s also a unique, and sometimes deadly, language of love.

Photos from the EarthSky community

An image of the night sky featuring M6 and M7 near Shaula and Lesath. In the foreground, firefly light trails appear in the silhouette of trees and over water.
Star clusters called Messier 6 and 7 shine near Scorpius’ stinger stars, Shaula and Lesath, while fireflies dance in the foreground on a summer night in Yellowwood Lake, Indiana. Image via Zolt Levay.
Fuzzy glowing line of hundreds of distant fireflies just above long grass in a field.
“Fireflies on top of the wave of grass and overflowing. Biggest firefly show in years,” said Eileen Claffey in West Brookfield, Massachusetts, June 2015.
Glowing yellow dotted lines in the air above a meadow.
Here’s a 30-second exposure from astrophotographer Tom Wildoner. Astrophotographers often capture fireflies when trying to photograph the night sky.
Night sky with yellowish streaks and one small, straight, narrow white streak.
You can see what looks like trails made by fireflies, via long-exposure photography from Jack Fusco Photography. There’s also a single meteor in the upper left of this photo. See it? It’s straighter than the firefly trails.
Nighttime landscape with many fireflies, appearing as short greenish-yellow streaks against dark evergreen trees.
Fireflies via Fiona M. Donnelly in Smiths Falls, Ontario.
Many concentric arcs of light spotted with short yellow glowing streaks.
Fireflies and star trails via Michael A Rosinski.
A small glowing lightning bug in a glass jar.
Did you ever do this? Image via Flickr user jamelah e.

Bottom line: Fireflies – aka lightning bugs – light up because of a chemical reaction between an organic compound in the fireflies’ abdomens – called luciferin – and the air.

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July 19, 2021

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