
Sight at Ochanomizu (1880) by Kiyochika Kobayashi. Source.
There are many places in Japan which are famous for fireflies, -- places which people visit in summer merely to enjoy the sight of the fireflies. Anciently the most celebrated of all such places was a little valley near Ishiyama, by the lake of Ōmi. It is still called Hotaru-Dani, or the Valley of Fireflies. Before the Period of Genroku (1688-1703), the swarming of the fireflies in this valley, during the sultry season, was accounted one of the natural marvels of the country. The fireflies of the Hotaru-Dani are still celebrated for their size; but that wonderful swarming of them, which old writers described, is no longer to be seen there. At present the most famous place for fireflies is in the neighbourhood of Uji, in Yamashiro. Uji, a pretty little town in the centre of the celebrated tea-district, is situated on the Ujigawa, and is scarcely less famed for its fireflies than for its teas. Every summer special trains run from Kyōtō and Ōsaka to Uji, bringing thousands of visitors to see the fireflies. But it is on the river, at a point several miles from the town, that the great spectacle is to be witnessed, -- the Hotaru-Kassen, or Firefly Battle. The stream there winds between hills covered with vegetation; and myriads of fireflies dart from either bank, to meet and cling above the water. At moments they so swarm together as to form what appears to the eye like a luminous cloud, or like a great ball of sparks. The cloud soon scatters, or the ball drops and breaks upon the surface of the current, and the fallen fireflies drift glittering away; but another swarm quickly collects in the same locality. People wait all night in boats upon the river to watch the phenomenon. After the Hotaru-Kassen is done, the Ujikawa, covered with the still sparkling bodies of the drifting insects, is said to appear like the Milky Way, or, as the Japanese more poetically call it, the River of Heaven.
From Lafcadio Hearn, "Fireflies", in Kottō (1902), pages 142-4.