Patchy emergence of the mass-emerging mayfly, Ephoron shigae Takahashi (Ephemeroptera: Polymitarcyidae) in the Yahagi River in central Japan

摘要

The mass emergence of Ephoron shigae Takahashi was observed to last one to four days on Koshido Bridge in the main stream of the Yahagi River (Table 1; Ban et al. 1994). In the survey of the mayfly in the river, however, the mass emergence seemed to finish in one day. In the Oto River, one branch of the Yahagi River, 151-225 subimagos/m~2 were collected by emergence trap on 12 September 1993 (Ban et al. 2000a). Because the larval density of this species is limited by the territorial habit of the larvae, the maximum density was estimated to be 150-250 individuals/m~2 from the field survey in the Oto River and theoretical model (Ban et al. 1994). Because the density estimated by the emergence trap was almost equal to the estimated maximum density of the larvae, it is reasonable to expect the mass emergence of the area to be complete in one day. Drastic decrease of the larval density by one day of emergence was also recognized in the main stream, which suggests a one-day completion of emergence, although quantitative data are not available for the change of the larval density. The question is why the mass emergence continues for some days on the bridge while it is complete in one day in the river.