Japanese Rivers

Naoshi C . WATANABE , Koukichi HATTA, Kazuo HISAEDA

Kazuaki HOSHI and Shin-ichi ISHIWATA

ABSTRACT

Seasonal and diurnal pattens of emergence were compared in

unisexual and bisexual populations of Ephoron shigae (TAKAHASHI)

from four rivers, including two near the northern and southern periph-

ery of the species range. During the emergence periods in 1991 and
1992 winged mayfiies were collected every day in the same way in the

four rivers. Seasonal timing and synchrony of adult emergence varied

depending on the locality and year. The daily start of emergence

gradually became earlier with sunset during the emergence period,

except for an unusually early emergence on a heavily rainy day,

suggesting that light intensity may affect the time of emergence. In

the bisexual populations, males began to emerge earlier than females.

Females in the unisexual populations began to emerge as early after

sunset as males did in the bisexual populations. Sizes of winged stages

gradually decreased during the emergence period : hypotheses of

decreasing size for cohorts developing under increasing temperatures

cannot be applied to E. shigae which spends its later nymphal stages

under temperatures fluctuating around the highest level of the year.

Key words : adult size, Ephoron shigae, timing of emergence

INTRODUCTION

Mass emergence in mid-September of a burrowing mayfly, Ephoron shigae

(TAKAHASHI) , has been reported almost every year in the mass media from

some Japanese rivers, because of its spectacular appearance and its interfer-

ence with traffic. WATANABE and ISHIWATA (1997) reported that this species

is distributed in rivers on Honshu, Shikoku and Kyushu from ca. 39'N

(Kitakami-gawa) to ca. 33'N (Banjo-gawa) , and includes unisexual (no or

few males) and bisexual populations . To elucidate the factors affecting the

seasonal and diurnal timing of emergence , we compare two bisexual and two

unisexual populations of E. shigae in a latitudinally and longitudinally broad

range .