The life cycle and growth ofIsoperla grammatica andI. difformis (Plecoptera) in southernmost Sweden: intra- and interspecific considerations

Studies on the life cycle and growth ofIsoperia grammatica andI. difformis from eight southern Swedish streams demonstrated that the former species shows greater between-stream differences than the latter species.I. grammatica larvae had typically two periods of rapid growth, autumn and spring, whileI. difformis larvae grew mainly in autumn-winter. In one of the streams, however,I grammatica grew rapidly throughout all winter. Maximum larval size differed between streams, and so did the timing of the first appearance of nymphs and nymphal maturation. Temperature was probably the most influential factor explaining all these differences. While sexual dimorphism inI. grammatica is slight but significant, it is very prominent inI. difformis. The sizes of larvae overlapped little between species as well as sexes suggesting that intra- and interspecific competition was, or had been, important. InI. grammatica this reduced overlap was expressed in several size-classes of males and females successively replacing each other, resulting in a meandering pattern of the size distribution charts. There were additional indications of possible interactions, such as a significant negative correlation between the densities of the two species.