Dispersal tendency of the adults of the water strider, Metrocoris histro, was clarified along a stream by the mark-recapture method, over a two-month's period Phenetic differences among members of 6 local populations from 6 streams within 3 rivers was estimated using the apterous males by multivariate morphometrics. Although the adult water striders generally seemed to be sedentary within a pool, the migrants showed a downstream bias in dispersal distance and in numbers of insects moving. Discrimination analysis showed that 67%-100% of the local group members were correctly classified into the original groups, and misclassification only occured among members sampled within a radius of 8-km. Thus the present result indicated that morphological similarity among members sampled from neighbor streams was higher than that among members sampled from the distant streams in a same river. Based on these results, patterns of dispersal and phenetic differentation of M. histro were discussed.
