Imagine walking into a dark park on a warm summer night. You step onto a narrow wooden path. The town around you has turned off its streetlights. Your eyes slowly get used to the dark. Then you see a single green dot floating above a stream. A second later, another green light appears. Soon, hundreds of bright green lights float through the trees and over the quiet water. This happens every June in a small town called Tatsuno, located in the mountains of Japan.
Tatsuno is home to one of the most famous firefly events in the world. People call it the Tatsuno Hotaru Matsuri. In Japanese, "hotaru" means firefly and "matsuri" means festival. Every summer, thousands of families travel to Tatsuno to watch thousands of fireflies light up the night air.
Meet the Genji Firefly
The insect that creates this display is the Genji firefly. Its scientific name is Luciola cruciata. It is larger than most fireflies found in North American backyards. An adult Genji firefly grows to about 1.5 to 2 centimeters long. That is about the length of a small paperclip. It has a dark body with a pink mark on its upper back.
Genji fireflies are famous for their bright yellow-green glow. Male fireflies fly above streams and rivers. Females rest on plant leaves near the water edge. The males flash their lights as they fly to call out to the females. If a female likes a male's signal, she flashes back from her perch on a leaf.
How Tatsuno Saved Its Fireflies
The story of Tatsuno's firefly park began more than one hundred years ago. In the early 1900s, Japan built new factories, roads, and farms. Trees were cut down along riverbanks. Factories and farms washed dirty water into local streams. Soil washed into the rivers, making the clear water muddy and slow.
Firefly populations dropped across Japan. In Tatsuno, local residents noticed that fewer fireflies flew near the Tenryu River each summer. They loved their glowing summer visitors and did not want them to disappear. In the 1920s, the people of Tatsuno decided to take action to save their fireflies.
Local residents worked together to build special water channels next to the main river. They dug shallow canals filled with clean, flowing water from mountain streams. They planted tall trees and shrubs along the banks. The shade kept the stream water cool during hot summer days. The thick trees also blocked wind and kept out artificial light from nearby roads.
Underwater Firefly Larvae and Freshwater Snails
To understand why clean water mattered so much, you have to look at how Genji fireflies grow up. A firefly goes through four main life stages: egg, larva, pupa, and adult. Most fireflies in North America spend their larva stage crawling in damp dirt, leaf piles, or rotting logs. Genji firefly larvae are very different. They live completely underwater.
After a female Genji firefly lays her eggs on moist moss along a stream, the eggs hatch into tiny larvae. These larvae drop right into the water. They live on the stream bed for almost a full year. They crawl along the gravel bottom looking for food.
Genji firefly larvae are picky eaters. They feed almost entirely on one specific animal: a small freshwater snail called kawaniwa in Japanese. Its scientific name is Semisulcospira libertina. These snails need clean, oxygen-rich water to live. They eat tiny algae that grow on rocks in clear, flowing streams.
When streams become polluted or muddy, algae cannot grow on the rocks. The snails die. Without snails to eat, the firefly larvae starve and die too. By digging clean water canals in the 1920s, the people of Tatsuno saved the snails. When the snail population grew, the Genji fireflies came back in huge numbers.
What Visitors See at the Festival
Today, this special area is called Matsuokyo Firefly Park. It is a protected haven for fireflies. On peak nights in mid-June, observers can count over ten thousand fireflies glowing at the same time.
The Tatsuno Hotaru Matsuri lasts for nine days every June. The festival feels very different from a noisy city fair. There are no bright neon signs or loud carnival rides inside the viewing area. Instead, the park is kept as dark as possible.
The town turns off nearby streetlights. Local volunteers stand at park entrances to guide visitors. People must follow strict rules to protect the insects. No flashlights are allowed along the paths. Visitors cannot use camera flashes or light up their phone screens. White light startles fireflies and makes it impossible for them to see each other's glowing signals.
Visitors are also forbidden from bringing jars or catching nets. Every single firefly must remain in the park. Catching an adult firefly stops it from mating and laying eggs for the next generation.
Flashing in Synchrony
As you walk along the dark trails, you notice something special about how Genji fireflies flash. They do not flash at random times. Male Genji fireflies fly together and synchronize their flashes, glowing in a shared rhythm.
Scientists have studied these flash patterns across Japan and discovered a fascinating detail. In western Japan, male Genji fireflies flash about once every two seconds. In eastern Japan, they flash slower, about once every four seconds.
Tatsuno is located right in the middle of Japan, where the eastern and western firefly populations meet. In Tatsuno, some fireflies flash every three seconds. Others can switch their timing depending on temperature or nearby males. Scientists are still studying how these different timing groups interact. It shows that even well-known firefly species still hold mysteries waiting to be solved.
Firefly Watching Around the World
The Tatsuno Firefly Festival shows what happens when a community protects firefly habitat over many decades. But you do not need to travel to Japan to watch fireflies or help them survive. Fireflies live on almost every continent except Antarctica.
On our Alitaptap map, citizen scientists log local firefly sightings every day. In late July 2026, observers reported Common Eastern Fireflies (Photinus pyralis) across North America. Sightings came from Beauchamp Road in Berlin, Maryland, and South 51st Street in Greenfield, Wisconsin. Members logged them in Sagamore Hills Township and Bay Village in Ohio, as well as locations across Michigan, Pennsylvania, North Carolina, Kansas, Illinois, and Texas.
Over in Europe, members recorded European Glow-worms (Lampyris noctiluca) in Bradenham, High Wycombe in the United Kingdom, as well as in Poland and Belgium. Meanwhile, Canadian observers logged Black Fireflies (Lucidota atra) in Tiny, Ontario, as well as sites in Quebec and Nova Scotia. Another member in Ohio spotted Say's Firefly (Pyractomena sayi).
Each species has different needs. Common Eastern Fireflies need damp soil and grassy fields. European Glow-worm females cannot fly, so they need undisturbed grassy banks where they can shine for flying males. Black Fireflies fly during the day and use scent signals called pheromones instead of light to find mates. Say's Fireflies appear early in summer and flash high up in tree canopies.
How You Can Protect Fireflies at Home
You can use the lessons from Tatsuno to protect fireflies in your own neighborhood. You do not need to dig a river canal to help. Small actions at home make a big difference for local insects.
First, reduce artificial light at night. Porch lights and garden spotlights confuse fireflies. Turn off outdoor lights when you do not need them. If you need light outside, cover your flashlight with red paper or use a red LED bulb. Fireflies cannot see red light as easily as white light.
Second, protect damp soil and leaf litter. Firefly larvae need moist ground to hunt for snails, worms, and slugs. Leave a small patch of your yard un-mowed, or leave a pile of autumn leaves under trees. This provides shelter for young fireflies all year round.
Third, avoid spraying chemicals on your lawn or garden. Chemical bug sprays and weed killers kill firefly larvae and their prey. Natural gardens with native plants help fireflies thrive naturally.
Finally, share what you see on Alitaptap. Note the date, location, and flash pattern when you spot a firefly. Every report builds a clearer picture of where fireflies are thriving and where habitats need protection.
Whether you are walking along a quiet stream channel in Tatsuno or watching your own backyard on a summer evening, seeing fireflies glow is an unforgettable experience. By protecting dark skies and clean habitats, we help make sure those green lights keep shining for years to come.