Regardless of the photoperiodic condition experienced by nymphs ofGerris lacustris latiabdominisMiyamoto, a long-day photoperiod of 14:10 (L:D) after the last ecdysis induces stronger positive phototaxis from day 10 or day 17 in females, and from day 26 or day 29 in males than does a short-day photoperiod (10:14 [L:D]). The abrupt reduction in the photophase from 15.5–15.25 h (including twilights) in the field to 14, 12, or 10 h, weakens the strong positive phototaxis of adult specimens in the summer. However, an abrupt change from ≍15.5 h (including twilights) in the field to 18 or 6 h has no effect on positive phototaxis. In the field, weakening of positive phototaxis in July and August is induced by the decrease in daylength after the summer solstice. This weakening appears to be an adaptation to rice paddy habitats in which water surfaces rapidly disappear during August and early September in Kochi. It is associated with early migration to diapause sites, which reduces habitat overlap and the possibility of competition with the larger species,Gerris paludum insularisMotschulsky.