Effects of temperature and light on the hatching of overwintering eggs in three Japanese Sympetrum species

The aim of the present study was to obtain quantitative information on egg hatching with respect to temperature and light to clarify the effect of cultivation methods on three Japanese Sympetrum species. Eggs of three Sympetrum species were collected on October 2005 at Akita prefecture located at north of Japan, and the eggs had been laid on soil surface of paddy field till April 2006. The eggs (3 trays with 50 eggs each) were held under four constant temperatures (8, 13, 18 and 23°C) with a photoperiod (L:D 14:10; relative light intensity 3,000 Lux) and 23°C in darkness. No S. infuscatum eggs, but 67 and 60% S. frequens and S. darwinianum hatched in constant darkness. This result suggests that S.frequens and S.darwinianum do not require light for hatching, but S.infuscatum requires light for hatching. Eggs of S. darwinianum and S. infuscatum did not hatch at 8°C. In S. frequens, some eggs hatched but the hatching rate was significantly lower at 8°C than at higher temperatures (P < 0.05). At higher temperatures, the hatching rate did not differ significantly for three Sympetrum species. At 13°C, S. infuscatum hatched fastest, 0.18 for S. infuscatum, 0.11 for S. darwinianum and 0.08 for S. frequens. The mean head width of second stadium larva of S. frequens, S. darwinianum and S. infuscatum were 0.4 ± 0.01, 0.4 ± 0.01 and 0.6 ± 0.01 mm (SD), respectively. These results suggest that S. infuscatum may have a competitive advantage over S. frequens and S. darwinianum under conditions that favor S. infuscatum through hatching speed.