Egg development, nymphal growth and life cycle strategies in Plecoptera

Interspecific and intraspecific differences in egg development, nymphal growth and the life cycles of Fennoscandian stoneflies are examined in relation to zoogeographical aspects and evolutionary trends. Data on other European and North American species are also considered.
Egg development shows considerable diversity, the main categories being ovovivipary, egg development with diapause and non-diapause development. Egg development in species in the non-diapause category can be classified as eurythermal, cold stenothermal or warm stenothermal. Temperature has a marked effect on nymphal growth of some species but not others. The distribution of several species can be explained by their temperature relationships, either in the egg or nymphal stage. Several species modify their life cycle to fit local climatic conditions. In some species, such as Leuctra hippopus , environmental selection has been so strong that they have probably recently evolved into subspecies and/or species in northern Europe.
Morphological and ecological variations in the Plecoptera have facilitated their colonization of the variety of freshwater biotopes that occur in Fennoscandia.