Information about Genji Fireflies

Information about Genji Fireflies

最終更新日:平成20年6月9日(月曜日) 14時51分



ゲンジボタルの卵 径0.5mm

ゲンジボタルの卵 径0.5mm



On Fireflies
Fireflies are insects which belong to the beetle family. There are about 2000 kinds of fireflies in the world, and about 40 kinds of them live in Japan. Most fireflies spend their lives on land and eat a member of the snail family. Larvae of Genji fireflies and Heike fireflies in Japan spend their lives in the water and eat freshwater snails, which makes them quite unique fireflies in the world.



●Genji Fireflies
Genji fireflies are indigenous to Japan. They had lived by various streams throughout the country and used to be quite familiar to the Japanese. Because these fireflies spend the most part of their lives in the water, however, quite a number of places have become uninhabitable for them due to the water pollution. Still, Genji fireflies flitting around a riverside in the gathering dusk would be typical fireflies of Japan.

The Life of Genji Fireflies

A mated female will deposit approximately from 500 eggs to 1000 eggs. Each egg has a diameter of 0.5 millimeters. Females often come to a riverbank covered with moss and place their eggs there one after another. Lots of eggs could be seen there in the season. Firefly eggs usually hatch in a month. The eggs deposited in late June will hatch in late July. Genji Firefly eggs are yellow at first and change their colors into black as the days pass. Their eggs are bioluminescent, and they glow dimly if stimulated.


Firefly larvae measure about 1.5 millimeters in length. They emerge from their eggs at night, go into the water, and grow up, eating thiaridal snails. The larvae eat small thiaridal snails when they are small in size. When they get bigger, they eat the bigger ones. In winter, firefly larvae become much less active and stay at the bottom of a river. In spring, they become quite active and the full grown larvae climb up the riverbank all at once, gleaming on a rainy night in late April.


When Genji larvae climb up the riverbank, they burrow into the ground and construct a mud chamber in the soil, pupating. It takes about 5 weeks for pupation and the pupal stage continues for 2 weeks or so. Genji firefly pupae are yellow at first while their compound eyes and tegmina turn black and light-producing organs are complete during the pupal stage. At this stage, they glow a lot. 2 or 3 days after emerging, the adult firefly gets out of the mud chamber and comes up to the ground surface.


An adult Genji firefly in Tatsuno can be seen in mid or late June. A female is approximately 2 centimeters long while a male is a bit smaller than that in size. A light-producing organ, which is called the 'lantern', is located in the abdomen. The female’s lantern is 1 segment while the male’s lantern consists of 2segments. Adult Genji fireflies live for about 10 days and reproduce during this time. On a warm windless night, a female place the eggs on a riverbank covered with moss while a male fly over the stream.


○Thiaridal Snails – The Food for Fireflies
Thiaridal snails are freshwater snails living in a river. In Tatsuno, they produce their babies from June to the end of September. A big mother snail could produce 10 to 20 baby snails with the size of 1 millimeter in a night. Thiaridal snails usually eat diatoms attaching to the stones at the bottom of a river. To recreate an inhabitable environment for fireflies, it is the most important thing to make the rivers clean so that more and more diatoms and thiaridal snails could live there.



●Genji Fireflies and Heike Fireflies
While Genji firefly larvae grow eating thiaridal snails in a river with the clean water, Heike firefly larvae grow eating such snails as pond snails(Monoara-gai) or mud snails (Hime-tanishi) in ponds or rice fields. They are rather resistant to water pollution. To tell the difference between Genji fireflies and Heike fireflies, it is useful to remember that a Genji firefly larva has a mark like a cross(+) on its back while a Heike firefly larva has a mark like a vertical line.




A Firefly Larva Eating a Thiaridal Snail


Glowing Firefly Larvae


A Genji Firefly Pupa


An Adult Depositing an Egg

左側:メス 右側:オス

Left: A Female Right: A Male