Summary 1. The stonefly Nemurella pictetii has a highly variable life cycle and is semi- and uni-, but also plurivoltine in different geographical areas. 2. The influence of the type of food on growth and maturation of larvae was studied in laboratory experiments. Food preference was also assessed. 3. Larvae of N. pictetii are strongly opportunistic feeders utilising a broad range of food sources. However, biofilm was preferred over detritus (conditioned leaves), green algae and animal food. The highly mobile larvae quickly found the preferred food. Larvae fed on biofilm grew and developed significantly faster than larvae fed on conditioned leaves. 4. Larval growth and maturation in experiments as well as in the field differed with available food qualities. Phases of faster growth and maturation corresponded well with maximum availability of biofilm in the field. This contributes to the interpretations of differences in life cycles in the field

Background: Considerable variation of life cycle duration in given insect species has been frequently recorded. Splitting of populations into cohorts with different life cycle lengths may occur, sometimes even between siblings from the same batch. Larval populations of the stonefly Nemurella pictetii in central Europe regularly split into a very fast developing and a normal univoltine cohort, leading to partial multivoltinism. The causes for such variation remain unknown but presumably act on the larval stage in which most of the life cycle is spent. We therefore studied possible effects of intraspecific competition on growth and development of larvae in the laboratory.