TABATA, RYOICHI, Laboratory of Animal Ecology, Division of Zoology, Department of Biological Sciences, Graduate School of Sciences, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan (present address: Lake Biwa Museum, 1091 Oroshimo-cho, Kusatsu, Shiga 525-0001, Japan).