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Abstract

Eggs of the stoneflies Perlodes microcephalus, Perlodes dispar and Perla burmeisteriana were incubated at constant temperatures between 4 and 18°C and in different photoperiods. Hatching success of P. microcephalus was high at all temperatures. P. dispar and P. burmeisteriana did not hatch at 4°C. Larvae hatched, especially at low temperatures, in pulses peaking at 100, 300 and 600-700 days after oviposition. In Perlodes microcephalus, viable larvae hatched even after 1000 days. The egg incubation periods of all species were strongly affected by temperature (most in P. burmeisteriana) but were not influenced by photoperiod. The otherwise very similar Perlodes microcephalus and P. dispar were clearly segregated by their egg incubation periods. Additional taxonomic features for distinction of Perlodes males are given. According to the temperature requirements of their eggs all species appear to be well adapted to their natural environment. The delayed hatching of some larvae may have high survival value through risk spreading under adverse conditions (cold years or industrial pollution).