2 subfamilies (one contains a single genus), with 35 spp. in 6 genera in our area(1)
and 106 spp. in 13 genera worldwide(2)
adult length usually <15 mm
Adult: dark brown to black with or without diffuse bands on wings; some species have yellowish or reddish markings on head, thorax, and tip of abdomen; forewing with forked vein rising from basal anal cell, 3 or fewer costal cross veins, and two adjacent rows of broad rectangular cells of similar size (resembling rungs of a ladder) near base of wing; antennae very long; 3 ocelli
Nymph: dark brown to black; second tarsal segment about as long as first (a diagnostic feature in nymphs and adults)
holarctic; throughout NA(2)
where the water conditions are right (in warmer areas, usually at high elevations)
Nymphs develop in cold, clear running water, usually in large streams and rivers.
Adults are often attracted to bridges over streams. Some species rest on fence posts and snow on warmer days of late winter.
Late fall to early spring (mostly January to April)
They're usually described as "shredders", which means they eat more-or-less whole living or dead plant material, or "detritivores", which means they eat pieces of broken-down organic matter. Adults and nymphs are plant feeders.
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. They also provide food for trout - though not as much as species active when trout are themselves more active in warmer parts of the year.
Adults of Small Winter Stoneflies
(Capniidae) are also small, dark, and present in winter, but their second tarsal segment is much shorter than the first, and they lack two adjacent rows of broad rectangular cells of similar size (resembling rungs of a ladder) near base of wing; instead, they have a few elongate cells of various sizes and not arranged in rows.