Imagine standing in a dark forest in Tennessee. You are surrounded by tall oak and hemlock trees. The sun set an hour ago. At first, you see nothing but dark shadows. Then, a single tiny light blinks near the ground. A second light flashes right next to it. Within three seconds, hundreds of yellow-green lights flash together all around you.
Then, just as quickly, everything goes pitch black.
You wait in the dark. You count slowly in your head: one, two, three, four, five, six. Suddenly, the whole forest lights up again at the exact same time. The trees look like they are decorated with pulsing lights. This is the nightly show at Elkmont, a mountain valley inside Great Smoky Mountains National Park.
Every year in late spring, thousands of people travel to Elkmont. They come to watch a special insect called Photinus carolinus. Most fireflies in North America flash whenever they want. Each bug has its own rhythm. But this species does something rare. The males team up and flash at the exact same second.
The Secret Code of the Flashing Light
To understand why this show happens, you need to know how fireflies talk. Fireflies use light instead of sound. Male fireflies fly through the air and flash a pattern. Female fireflies sit near the ground in the leaves or grass. If a female likes the flash pattern of a male, she flashes back.
For most fireflies, this conversation is easy to spot. Take the Common Eastern Firefly, which many people see in backyards during July and August. The male flies up, makes a glowing yellow letter "J" in the air, and turns off his light.
The synchronous firefly at Elkmont uses a completely different code. The male Photinus carolinus flashes quickly six to eight times in a row. Flash, flash, flash, flash, flash, flash. Then he turns his light off and stays dark for six to eight seconds.
When thousands of males do this together, the whole woods light up. Scientists call this synchronous flashing. Synchronous simply means happening at the exact same time.
During those few seconds of darkness, every male stops glowing. That brief silence gives females on the ground a chance to answer. A female watches for the group burst. Then she blinks back twice from the leaf litter. If the males kept glowing continuously, her faint reply would get lost in the glare.
How a Local Secret Became Famous
People living near the Smoky Mountains knew about these glowing bugs for a long time. For decades, families in East Tennessee visited Elkmont in late spring to watch the lights. They thought all fireflies flashed together like that. It felt like a cozy local secret.
That changed in the early 1990s. A woman named Lynn Faust grew up visiting a family cabin at Elkmont. She loved watching the fireflies every spring. As an adult, she read a science book about fireflies. The book stated that synchronous fireflies only lived in Southeast Asia, along riverbanks in places like Malaysia and Thailand.
Lynn knew that was not true. She had been sitting under synchronous fireflies in Tennessee her whole life!
She wrote a letter to a firefly scientist named Jonathan Copeland. At first, scientists were unsure. They thought she might be seeing an optical illusion. Sometimes, if you look at many blinking lights at once, your eyes trick you into thinking they match up.
Dr. Copeland came to Elkmont to check for himself. He brought cameras and light sensors into the woods at night. When he measured the flashes, he found that Lynn was completely right. The beetles were indeed flashing in near-perfect rhythm. In 1992, scientists published a paper about the Elkmont fireflies, and the news spread worldwide.
What Visitors Experience Today
Today, Elkmont is one of the most famous firefly watching sites on Earth. So many people want to see the show that Great Smoky Mountains National Park created a system to protect the bugs and their habitat.
Each spring, the park holds a ticket lottery. Thousands of families enter, but only a few hundred cars are chosen for each night of the peak week. Park rangers drive visitors to the viewing site on shuttle buses. This keeps traffic off narrow mountain roads and stops bright headlights from blinding the fireflies.
Visitors walk along a dark trail into the woods. Everyone carries a flashlight covered in red plastic wrap or red cellophane. White light from flashlights or smartphones stops fireflies from flashing. Fireflies cannot see red light well, so red light lets humans walk safely without disturbing the bugs.
When people arrive at the viewing spot, they sit quietly on blankets or folding chairs. They turn off all their electronics. As darkness fills the forest, the show begins.
It starts around 9:30 PM with a few scattered flashes. By 10:00 PM, thousands of males fly through the dark trees. Hundreds of yellow-green flashes pulse six times, then drop into total darkness. The crowd sits in silence, listening to the mountain creek and watching the dark woods glow.
Why Fireflies Sync Their Lights
Why do these beetles sync their flashes? Scientists have two main ideas.
The first idea is about dense forest leaves. Elkmont sits in a mountain valley with thick trees and heavy bushes. If every male flashed at his own random time, the forest would look like a chaotic mess of tiny blinks. A female on the ground would have a hard time picking out individual signals through the leaves. By flashing together, the males make a bright signal that penetrates the thick foliage.
The second idea is about female eyesight. A female firefly has eyes that respond to brightness. When hundreds of lights flash together, the total brightness makes it much easier for her to notice that males are nearby.
Scientists also noticed an interesting wave pattern. The synchronization does not happen instantly across the entire forest. Instead, one male flashes, and his neighbors copy him milliseconds later. This creates a ripple of light that moves through the woods like a wave of fans in a sports stadium.
Late Summer Fireflies in Your Neighborhood
The Elkmont firefly season is short. It lasts about two weeks each year, usually ending by mid-June. After that, the adult Photinus carolinus fireflies finish mating, lay eggs in moist soil, and die. Their larvae hatch later in summer and live in the soil, eating tiny snails and soft insects.
Even though the Elkmont event happens in early summer, firefly season continues in many other places through August. Right now on Alitaptap, community members are logging sightings across North America and Europe.
In early August, watchers recorded Common Eastern Fireflies in places like Nashville, Tennessee; Durham, North Carolina; Wheeling, West Virginia; Brooklyn, New York; and Ann Arbor, Michigan. While these backyard fireflies do not flash in synchronized groups like the Elkmont beetles, they put on their own great show near lawns and forest edges every warm evening.
Over in Europe, users on Alitaptap and iNaturalist are spotting European Glow-worms in France, Germany, and Switzerland. Unlike the flying males at Elkmont, female glow-worms do not fly at all. They sit on grass stems and glow softly to guide flying males down to them.
Every species has evolved its own way to find a mate in the dark. You can help protect them by keeping your yard dark at night and logging your sightings on Alitaptap!