"The reason is probably to be found in Icelandic conditions. In Greenland and Northern Scandinavia, the pupa hibernates beneath ice during the winter, and hatches as a fly as soon as the ice melts. This happens in spring, as polar winters are continuous. Icelandic winters are variable. There can be a sudden rise in temperature in the middle of winter, with a thaw, then the temperature will drop again.
Under these conditions the pupa would hatch. The mosquito would then need to find prey from which to suck blood, then it would need several days for the eggs to mature, to meet a mate and lay the eggs in a pond or marsh. Changes in climate in Iceland are so rapid that the mosquito does not have sufficient time to complete its lifecycle. Under these conditions the pupa would not be mature when temperatures dropped again and ice formed on the ponds."
https://www.why.is/svar.php?id=5488
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