Nymphs climb out of the water and shed their skins to become adults. This is usually in late winter. The adults live just long enough to mate and lay their eggs. The nymphs hatch shortly and feed for a while before going into diapause (a hibernation-like state) from mid-spring until fall. Their main period of growth and activity is fall and winter.
The defining need of winter stonefly nymphs is for very high levels of oxygen in the water. Warm temperatures, excessive organic matter, and many pollutants all reduce oxygen levels. The result: they're only active in the coldest part of the year and are very sensitive to pollution.
Their main interest to humans is as an indicator species: you can tell that water is unpolluted if stoneflies live there.
Вот какая у нас в Висконсине чистая вода!