Abstract
SUMMARY. 1. The duration of egg incubation (Y) in Dinocras cephalotes and Siphonoperla burmeisteri was related to constant temperatures from 4 to 24°C, by the regression equations Y=2382 T −1, 402(r2=0.992, P<0.001) and y= 2683 T−1.667 (r2=0.994, P<0.001), respectively. No diapause was observed in either species.2. Egg incubation in D. cephaloles was slow and took 784.9±92.7 (mean ± SD) degree days between 12 and 20°C. significantly more than in S. burmeisteri(445±76.17 degree days: t= 7.44. d.f.=13,P<0.001).3. For D. cephalotes hatching occurred at temperatures between 12 and 24°C, and for S. burmeisteri between 8 and 20°C. The mean volume of the eggs of D. cephalotes was about 5 times greater than that of S. burmeisteri and the mean body lengths of the newly‐hatched nymphs were 1.13 mm and 0.95 mm respectively.4. This study shows that the freshwater fauna of northern Fennoscan‐ dia also contains species with warm stenotherm eggs. D. cephalotes. which is of a Mediterranean origin (Zwick, 1981a), may exist at the limit of its distribution in northern Fennoscandia.
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