Fireflies: How and why they light up

A glowing bug, with 6 legs and antennas, seen from underneath.
How fireflies glow: In essence, a chemical reaction in the beetle’s abdomen gives it its bioluminescence. Image via Cathy Keifer/

Whether you call them lightning bugs or fireflies, these beetles are a sign of summer. Indeed, you’re most likely to see them in warm weather, when rainfall has been plentiful. Many a child has spent a summer evening chasing them. And, with this in mind, maybe you’ve wondered: Why do fireflies light up? And how do they do it?

How fireflies light up

First, let’s talk about the how. The light of a firefly is a chemical reaction caused by an organic compound – luciferin – in their abdomens. As air rushes into a firefly’s abdomen, it reacts with the luciferin. Consequently, 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.

In addition, fireflies can regulate the airflow into their abdomens to create a pulsating pattern.

Why fireflies light up

First, fireflies light up for safety. 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. In fact, they eat so many fireflies that they themselves begin to glow.

Second, fireflies light up for romance. Male fireflies’ light signals their desire for mates. Also, willing females attract males with flashes of their own.

Lastly, fireflies light up to attract dinner. While each firefly species has its own pattern of flashing, some females imitate the patterns of other species. As a result, 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. Indeed, it’s also a unique, and sometimes deadly, language of love.

Photos from the EarthSky community

Curved green lines and concentric star trails above backyard grass and trees.
View at EarthSky Community Photos. | Dan Bush in Albany, Missouri, captured fireflies and star trails by means of a single-night photo composite on June 18, 2022. He wrote “This is a view from my aurora camera which is designed to record auroras and meteors in my northern sky. There were no auroras or meteors this evening but there wasn’t a lack of things to see. The circular arcs in the sky are circumpolar star trails, the horizontal white streaks in the middle of the image are the headlights from cars passing by, and the curved green/yellow streaks mostly seen against the moonlit landscape are from lightning bugs, or fireflies, depending upon which part of the world you live.” Thank you, Dan!
Short, curved yellow dotted lines and concentric star trails against twilight sky over dark hills.
View at EarthSky Community Photos. | Matthew Chin in Sha Lo Tung, Hong Kong, captured fireflies and star trails in one photo on August 29, 2017. Matthew wrote: “With limited places, we can see vast number of fireflies. I tried to have the light trails of the little fireflies and the light of stars in one photo.” Thank you, Matthew!
Bright curved dotted lines against dark twilight sky over grassy field.
Fireflies via Matt Pollock in upstate New York.
Milky Way above, glowing yellow, curved dotted lines against landscape with pond.
Here, star clusters 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 dashed lines in the air above a meadow edged by trees.
Here’s a 30-second exposure from astrophotographer Tom Wildoner. In fact, 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. Also, 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 very many short greenish-yellow dashed streaks against dark evergreen trees.
Fireflies via Fiona M. Donnelly in Smiths Falls, Ontario.
Many concentric arcs of light in sky, spotted with short yellow glowing streaks.
Fireflies and star trails via Michael A. Rosinski.
A small glowing yellow bug in a blue glass jar, in dim light.
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.

Enjoying EarthSky? Sign up for our free daily newsletter today!

June 8, 2022

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

Editors of EarthSky

View All