Abstract
Under identical rearing conditions, the individuals of the Hiroshima strain, the females of which were predominantly brachypterous within a broad range of nymphal density, showed a shorter nymphal period, but emerged to adults with a similar weight and shorter relative fore-wing length against head width, compared with the individuals of the Nagasaki-I strain, the females of which exhibited higher ratios of macropterous forms with increasing nymphal density. Both strains exposed to different combinations of photoperiod (10 and 16h) and temperature (20 and 25°C) showed similar responses in their developmental pattern: a short day-length slightly prolonged the nymphal period. Detailed studies on the Hiroshima strain showed the presence of a photo-sensitive period during the adult and egg stages, but the induction of a diapause-like prolongation of a specific stage could not be achieved regardless of the conditions to which they were exposed. The females of the Hiroshima strain were totally brachypterous when reared on rice plants before the heading period, but they partly exhibited macropterous forms on older plants, indicating that even highly stable populations like those of the Hiroshima strain could migrate as macropterous forms when fed on rice plants near the ripening stage. From these results, it was suggested that the considerable variations in the nymphal density and wing-form relationships in N. lugens collected in Japan were not caused by differences in the sensitivity to photoperiod, but were due to the difference in the migratory sources.
