Diversity in the Life History of the Dragonfly Sympetrum frequens (Odonata : Insecta)

Sympetrum frequens is a very popular dragonfly in Japan. However the details of the life history are not well known. The nature and diversity of the life history in the species are discussed. In most regions of Japan, imagines of the species emerge fairly synchronously in early summer mainly from paddy fields of lowlands and soon migrate to highlands, where they spend the summer. At the beginning of autumn, they return to lowlands for breeding. Thus they have extremely long prereproductive periods caused by reproductive diapause, lasting 2 to 3 months. This prolongation in the prereproductive period is likely to be seasonal regulation in order to postpone the start of their oviposition until the cool season when it becomes possible to overwinter in the egg stage. The migration to highlands by the diapause adults during this period seems to be so as to escape from hot summer of lowlands in Japan. The evidences suggest that the upper limit of their favourable temperature is around 22-23℃ and that the migration takes place at the localities where the mean temperature exceeds the limit in summer. In Hokkaido Island in the northern Japan, the life history pattern is scarcely known but some evidences suggest that no prolongation of the prereproductive period owing to reproductive diapause occurs. It is also unlikely that the migration to highlands occurs there because the mean temperature is usually below 23℃. Such non-diapause and non-migrating life history pattern is applicable to a number of small sized, dark-coloured S. frequens (highland type) which were found on some high mountains of Honshu (mainland of Japan). In lowlands, although most individuals emerge synchronously in early summer, a small number of individuals emerge later in mid summer up to autumn. The mid-summer emerging individuals probably need to migrate to highlands, but the autumn emerging individuals may not do because the mean temperature of lowlands will be soon or is already lower than 23℃. From mid-to late summer a small number of early matured individuals, wich are not highland type, are sometimes collected and their reproductive behaviour is also observed as unusual cases. These facts suggest that a part of the lowland populations does not enter reproductive diapause. I speculated that such non-diapause individuals are late emerging ones from lowland habitats because the highland type individuals and Hokkaido populations, which are non-diapause, are also late emerging ones. Whether or not entering reproductive diapause may be mainly controlled by photoperiodic conditions at emergence. It assumed that S. frequens has the following three different types of life history pattern. Type (1) is assumed for Hokkaido populations, the highland type individuals, and the autumn emerging individuals of lowland populations. Neither reproductive diapause nor the migration to highlands occurs among them. Type (2) is assumed for a part of the midsummer emerging individuals of lowland populations. They usually migrate to highlands but do not enter reproductive diapause. In unusual cool summer, they remain lowlands and breed there in mid-summer. Type (3) is assumed for the early summer emerging individuals of lowland populations. Both reproductive diapause and the migration to highlands occur. The life history pattern in which reproductive diapause takes place but no migration to highlands as known by Lestes sponsa in southern Japan has not been found in S. frequens.