Abstract
The whirligig beetle, Gyrinus curtus Motschulsky, 1866, is an aquatic insect endemic to Japan, inhabiting standing or slow-flowing waters from Hokkaido to Kyushu, and has been listed as an endangered species due to habitat degradation since the 1960s. The only reliable record of this species in Saitama Prefecture is based on incidentally obtained specimens from puddles in Kumagaya City, where no reliable habitat has been found. This survey revealed that the whirligig beetle inhabits the endangered sticklebacks “Musashi-tomiyo” habitat in Motoarakawa River, designated as a natural monument of the prefecture. The habitat of G. curtus originates from a spring and is stable in water quantity and temperature. This characteristic is similar to the environment in Fukushima Prefecture, where this beetle and different sticklebacks from “Musashi-tomiyo” are also found. In other words, both species (G. curtus and sticklebacks) are presumed to prefer similar environments.
