Overlapping distribution of two groups of the butterfly Eurema hecabe differing in the expression of seasonal morphs on Okinawa-jima island

In Japan, Eurema hecabe (L.) has two geographically separated groups (subtropical and temperate) differing not only in the color pattern expression of the wings associated with ovarian arrest, but also in the fringe color of the wings and host-plant utilization. In a field survey (Kato, 1999), it was found that individuals with different fringe color of the forewing (brown or yellow type) inhabit Okinawa-jima Island located in north of the subtropical region, and that fringe-color is linked with seasonal morph expression. In the present study, rearing experiments were done to investigate whether these sympatric individuals are different forms within one species or belong to different species. The results show that each population of brown- and yellow-types maintained its own seasonal morph response to photoperiod and temperature through three generations. Host-plant utilization by the larvae and the fringe-color also differed between them and was maintained. These findings strongly suggest that E. hecabe of Okinawa-jima Island is divided into two distinct populations, and that one belongs to the temperate and the other to the subtropical group.