Three species of Isoperla occur in Fennoscandia. The most common is I. grammatica, although it has not been recorded above the subalpine vegetation belt. I. difformis has a similar distribution, but is less common. I. obscura occurs mainly at high latitudes and altitudes, although it is also abundant in large rivers. Eggs were reared at temperatures of 4, 8, 12, 16, 20 and 24° C. I. difformis did not hatch at 4° C. I. grammatica at 4 and 8° C while I. obscura did not hatch at 16° C and above. Egg mortality was high in I. difformis at 24° C. Incubation period decreased with increase in temperature and varied between 28 and 68 days in I. grammatica and between 15 and 83 days in I. difformis. For I. grammatica and I. difformis, the relationship between the egg incubation period and the water temperature was linear on logarithmic scales. For I. obscura, no such relationship was found and the mean incubation period at all temperatures was 305 ± 57 (S.D.) days. Explanations are presented to explain these differences in egg incubation characteristics.
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