Winter Stoneflies

If winter travels take you along the banks of a clean, rocky stream on a warm afternoon, chances are good you’ll encounter stoneflies (order Plecoptera) seeking out others of their kind by crawling along riverbank snow. Not every stonefly can handle the depths of northeastern winters, but two families of stoneflies are adapted to emerge when snow coats our riverbanks. Large winter stoneflies in the family Taeniopterygidae, also called “willowflies,” often occur side-by-side with small winter stoneflies, or “snowflies,” in the family Capniidae. In shape, size, and color, the bodies of these two types of stoneflies resemble lengths of thick pencil lead with six leggy spindles that lift them off the snow, preserving what warmth their dark bodies harvest from winter sun.

As the common names suggest, large winter stoneflies are larger, but only barely so, and the two families overlap broadly in size. Large winter stoneflies top out at two-thirds of an inch, while small winter stoneflies may be as long as half an inch. To more reliably tell them apart, you’ll need a closer look at the wings.

Stonefly wings are clear with dark veins, like stained glass window joints, and the insects hold these wings flat over the length of their bodies. The wings of both types of winter stonefly cover the width of the insect’s body. Wings are attached at the thorax and extend past the end of abdomen. It can be challenging to see the subtle differences between the two families, and a zoomed-in photo can be helpful to distinguish one from the other. Large winter stoneflies’ wings have a series of short cross veins resembling rungs of a ladder. Small winter stonefly wing veins lack this ladderlike appearance. Stoneflies are generally poor fliers, and in cold conditions, they are more likely to be seen crawling than flying.

Regardless of family, all stoneflies have two prominent tail-like cerci at the back end that help distinguish them from mayflies (order Ephemeroptera), which most often have three cerci. Paired stonefly cerci persist into adulthood but are sometimes concealed beneath the wings. (In many Capniidae species, the cerci peek out from under the wings at the back end.) Mayfly adults hold their wings vertically over their bodies, like hands in prayer, which easily distinguishes them from stoneflies.

Adult stoneflies are the brief final stage of a life cycle that lasts a year or longer. Females lay hundreds or even thousands of highly textured eggs, each just larger than the period at the end of this sentence, and they secrete a sticky substance to attach the eggs firmly to submerged river rocks. Winter stoneflies lay their eggs between January and May with peak egg-laying in March. Species and habitat conditions affect how long it takes for tiny stonefly nymphs to hatch and begin foraging. While some stonefly eggs hatch after only a couple of hours, winter stonefly eggs may enter a state of suspended animation for weeks or even months until temperature and water flow conditions are right for hatching. The larvae can similarly enter an additional diapause if conditions become unfavorable and will resume feeding and growth when things improve. When streams dry out, eggs can persist, and larvae migrate down to find moisture until rain or snowmelt refills the stream.

Winter stonefly nymphs are aquatic and consume an array of foodstuffs including algae, diatoms, fungi, organic detritus including leaf particles, and even pollen that settles in stream pools. Some winter stoneflies will also prey on other insects, and many are opportunistic omnivores, eating whatever fits between their mandibles.

All this feeding allows for stonefly growth through several stages or instars. Nymphs feed, grow, and then reach a point where they shed their exoskeleton, expand and harden a new exoskeleton, and repeat. Winter stoneflies repeat this process as many as 33 times before the adult finally emerges from the exoskeleton of the last larval instar and comes ashore as a nonfeeding, terrestrial adult.

A male stonefly uses his abdomen to drum rhythmically, like a lovelorn Ringo Starr, to attract females. This drumming is inaudible to human ears, but travels rapidly through solid objects such as tree branches to be received by stonefly members of the opposite sex. Once mating is complete, females affix clumps of eggs to river rocks, completing the cycle. Adult stoneflies live only long enough to mate and will die soon after.

Stoneflies – winter and otherwise – are important indicators of clean and healthy rivers and streams. Organic pollution removes oxygen from stream water, and eroded soil, silt, and sand from clear-cuts or from urban or agricultural landscapes can foul stonefly gills and fill the nooks and crannies between river stones that comprise stonefly habitat. So, the more stoneflies – and the more different species of them – you find, the higher the likelihood of clean water.

Fly-fishing enthusiasts hoping to lure hungry trout from chilly winter waters might cast a “Rich’s Winter Stone” or “Pat’s Rubber Leg” fly on open water between icy patches. These flies are slim bodies wrapped on hooks with prominent legs, antennae, and tails – and perhaps good enough to tempt a hungry trout. Whether your winter excursions take you into or just close to flowing water, keep your eyes peeled along riverbanks and streamside vegetation for these resilient indicators of healthy riverine habitats.