Adult emergence patterns and body size of two mayfly species of Isonychia Eaton, 1871 from Hokkaido, Japan

We conducted an emergence survey of Isonychia (Isonychia) japonica Ulmer, 1919 and Isonychia (Prionoides) shima Matsumura, 1931 at four sites in the Izari-gawa River system, Hokkaido, Japan in 2015 and 2016 to investigate differences in emergence seasons between the species. The present study showed that I. (I.) japonica was widely distributed from up- to down-stream with abundant numbers in middle-stream, while I. (P.) shima was mostly distributed up-stream. The emergence season of I. (I.) japonica was longer than that of I. (P.) shima, but their emergence periods overlapped in August and early September, suggesting that their adults are not separated temporally. Body size comparison showed that, at the same sites, the emerged in August specimens of I. (I.) japonica were larger than those of I. (P.) shima.