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Organisms occurring in the temperate zones of the earth must have adapted to seasonal fluctuations of the climate characteristic of these regions. Under changing environmental conditions the organisms exhibit plastic reaction norms, for example life cycles of different length resulting from different development rates under particular climatic conditions. The stonefly Nemurella pictetii is a species with a highly variable life cycle. Its life cycle can differ between different localities in its large range of distribution, but also between individuals at some given locality. To investigate possible causes of this variability was the aim of the present work. To this end, larval development rates were analysed under different environmental conditions in laboratory experiments focusing on temperature, photoperiod, food quality and intraspecific competition. All of these environmental factors were shown to influence the development rate synergistically, or also antagonistically. Temperature exerts the greatest influence, rising temperatures leading to accelerated development. Opportunistic feeding enables N. pictetii to use a variety of food resources. High-quality food also accelerated development. However, when intraspecific competition interferes, splitting of the larval population into several cohorts may occur. The reaction of the larvae to photoperiod depends on temperature. Short days at high temperatures raised developmental rates whereas short days in combination with low temperatures retarded development. Such interactions additionally enhanced differences in development between individual cohorts. In summary, N. pictetii is characterised by its generalistic mode of life, including a high plasticity in development rate which allows fast and flexible reactions to changing environmental conditions. This way, N. pictetii can colonise large parts of Europe and can also adapt the number of generations per year to the particular local environmental conditions.