In two bog-dwelling dragonflies, Somatochlora alpestris and S. arctica, the influence of oviposition date and temperature upon duration of embryonic development was studied. Egg diapause was facultative. With advancing season, the proportion of diapause eggs increases from 0 to 37% in S. alpestris and from 0 to 18% in S. arctica. Eggs needed at least 17 to 38 days for development. Hatching curves were temperature-independent in nearly all experiments but developmental rate increased at higher ambient temperature. In S. arctica, responses of developmental rate to temperature differed in eggs laid on different dates. In S. alpestris, duration of egg development decreased as season progressed. The duration of egg development of non-diapause eggs and proportion of diapause eggs in S. alpestris and possibly in S. arctica may also both be a function of female age at the time of oviposition. The ecological significance of the different development patterns is discussed.