The Fireflies of Arimagawa River

The beautiful scenery of the early summer at Arima Onsen would have to be the emergence of the Sal Tree flowers at Nenbutsuji Temple and the fireflies ("hotaru" in Japanese) of Arimagawa River. For one month, swarms of fireflies amass at Arimagawa River at the end of May and truly fantastical scenes of illumination are visible at night as the fireflies flutter along the riverbanks until the end of June. Should you have a chance to visit Arima Onsen during this season, we heartily recommend that you come out to appreciate the beauty of this natural spectacle.

Japanese version

The Influence of the Resurgent Fireflies

The Arima Onsen Elementary School The Arima Onsen Elementary School

Some dozen years have passed since the Arima Elementary School began striving for an "Arima Onsen that is kind to people and a firefly elementary school." In cooperation with the Recycle Promotion Section of the Kobe City Environmental Bureau, the student council, and the Arima Onsen Tourism Association, the entire school body became involved in the breeding and stocking of fireflies in 1991 with the motto of "Let's beckon a return of the fireflies to Arima."

The Spot for Appreciation of Fireflies

Map for the spots for appreciation of fireflies
Click here to enlarge the map.

Two popular sightseeing spots near the Arima Onsen Station of the Rokko-Arima Ropeway are Tsuzumigataki Waterfall and Masu-ike Pond. The river flowing from the waterfall is called Takigawa River. Here the river runs into the Rokkogawa River near Nene-bashi Bridge, and this forms Arimagawa River. Passing beneath the Taiko-hashi Bridge near the Arima Onsen Station of the Kobe Railway, Arimagawa River flows under the Koen-bashi Bridge, which is near the Arima Elementary School, and the Otokura-bashi Bridge, which is further downstream. It continues along going north until it finally joins the upper parts of the Mukogawa River.

Those visiting Arima Onsen with the intention of appreciating the beauty of the fireflies can enjoy the sight of them at night on the riverbanks downstream from the Masu-ike Pond on Takigawa River and the area spanning from Koen-bashi Bridge to the Otokura-bashi Bridge.

Tsuzumigataki Waterfall Takigawa River The Otokura-bashi Bridge Arimagawa River

Etiquette for Appreciating Fireflies

Etiquette for Appreciating Fireflies

Fireflies in General


Fireflies can live anywhere near a river if the water is clean and not polluted by agricultural pesticides or synthetic detergent.

Types of Species

Around 2,000 species of fireflies can be found all over the world with 40 types found in Japan. Of the fireflies seen in Japan, the most common are the three types described below. The type of firefly commonly seen at Arimagawa River is the Genji Firefly (Luciola cruciata Motschulsky).

Comparison of the Three Types
TypeGenji Firefly
(Luciola cruciata Motschulsky)
Heike Firefly
(Luciola lateralis Motschulsky)
Hime Firefly
(Luciola parvula Kiesenwetter)
PhotographGenji FireflyHeike FireflyHime Firefly
Body length10-20mm10-18mm5mm
Illumination cycle2 second intervals1 second intervals0.5 second intervals
Luminescent-colorYellowYellowRose pink
HabitatSmall clean river, irrigation channelRice paddy, irrigation channel, small clean riverCedar, white cedar forest, etc.
Food for larvaMelanian snailSinotaia quadrata histrica (pond snail), Austropeplea ollula (Japanese ear snail), Physa acuta, etc.Allopeas clavulinum Kyotoense, Clausiliidae, etc.
Appearance (in general)Beginning of June to the end of JuneMiddle of June through the early part of JulyEarly June to late July
Number of eggs400-1000 eggs70-100 eggs50-100 eggs

Life Expectancy

Lifetime is about 10-14 days for both males and female after becoming imago (adult firefly).

Mode of Life

Firefly Development
Click here to enlarge "Firefly Development".

Fireflies cast off their skin as larva five times, go ashore, and cast off their skin once more after forming a cocoon. They spend nine months as larva on the river, go ashore in April when it rains, and make a cocoon in the ground. They become pupa and emerge around fifty days later. When they become imago (adult firefly), they don't eat and only come out at night.

Chance for Appreciation

Genji fireflies can often be seen on dark, cloudy, sultry evenings but are not active when temperatures drop. Peak times for these fireflies are 8:00-9:00 pm or in the dead of night from 12:00-1:00 am or 3:00-4:00 am.

Illumination Mechanism of Fireflies

Illumination Mechanism of Fireflies

The fantastical illumination emitted by fireflies involves the same mechanism for light as found in the oxidation reaction of a burning candle.
In the luminescent organ of the firefly, there is a luminous substance called luciferin that reacts to oxygen in the air to emit light.
Usually oxidation at the time of a reaction emits heat and light, but the enzyme luciferase within the luminescent organ of the firefly efficiently oxidizes without expending a great deal of energy and changes to light.
Furthermore the faint glow of light and darkness is linked to breathing. When it breathes, the membrane blocking the light is lowered, and light is extinguished.

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