Stoneflies (Order Plecoptera) are generally of lesser importance in Wisconsin and the Midwest but "out west", they are the "glory hatches" that anglers plan their trips around. The big stonefly hatches of the West, most notably the salmonflies (Pteronarcys spp.) and golden stones (family Perlidae), are two hatches many anglers - including myself - have chased out west. We fished the Giant Salmonfly (P. californica) in southern Colorado - though hitting them "just right" was a real challenge, we did catch enough of the hatch to make it interesting. In Wisconsin, there are some larger freestone rivers where Pteronarcys (probably P. dorsata) are common. The Yellow Sallies (Isoperla spp.), a smaller stonefly that is typically imitated with a #14 fly, is more common and widespread in the state. They tend to be a sparse mid-day hatch in late June but they draw up trout pretty regularly. And a number of "winter stoneflies" hatch during the winter, typically during the middle of the day on warmer winter days.
And, of course, the Bunyan Bug, stonefly #2, might be the most recognized fly by both fly anglers and non-fly anglers of a particular age due to the fly being the star of one of the most famed scenes of A River Runs Through It.
The stonefly life cycle is simpler than are the life cycles of caddisflies, midges, and mayflies; each of which go through additional transformations between the nymphal and adult life stages. Unlike midges and caddis, which have a pupal stage, and mayflies, which have subimago (dun) and imago (spinner) stages, stoneflies molt from larva (nymph) to adults. And unlike these other aquatic insect groups, stoneflies crawl to shore or emergent mid-stream rocks and woody debris and molt into adults (imagos) out of the water. A sure sign of a stonefly hatch is seeing a number of shed exoskeletons on shore and on mid-current rocks and wood.
Nymph
Stonefly nymphs are easily identified - and separated from mayfly nymphs - by the fact that they have thoracic gills compared to the abdominal gills of mayflies and by always having two tails (mayflies have 2 or 3 tails) which are generally much shorter and stouter than are mayfly tails. Additionally, stonefly nymphs tend to have a more elongate and rounder body than do mayflies so they are typically tied on 2x to 4x long hook shanks. Stoneflies are typically associated with heavy riffles or rapids in larger streams due to their high oxygen requirements. They are generally thought to be intolerant of pollution, however they have evolved to live in rather acidic environments - so long as there is sufficient oxygen. In Wisconsin, they are most common in larger freestone rivers which are often tanic stained and relatively low in pH.
I am very rarely fishing stonefly nymphs in Wisconsin, however I would probably do so more often if I fished Northern Wisconsin freestone rivers like the Wolf, Peshtigo, Namekagon, and other large trout rivers more often. During warmer winter days, I may fish a small, elongate nymph to imitate the Capniidae stoneflies I expect to see hatching by mid-day. These are the small (#18) dark stoneflies you see from January through about spring break. My top choices during this time are either a small, slender, dark hare's ear type nymph or a dark softhackle. In either case, I am mostly fishing them around shore or larger midstream obstructions, trying to imitate the migrating nymphs and those that are dislodged from substrates during this migration. The Driftless Area tends to see a pretty good winter stonefly hatch but a sparse or absent early brown stonefly hatch (Genus Strophopteryx). The last stonefly of importance in Driftless streams are the Isoperla spp. - yellow sally - stoneflies of mid-June. However, I can say that I rather rarely imitate these in their nymphal stage. It is generally a sparse and unpredictable hatch and by the time I recognize them, there are adults on the wing and on the water.
While dead drifting nymphs certainly works and probably imitates a dislodged stonefly pretty well, a more actively swung or twitched fly is generally more effective. Wet flies - like the Talasek's Killer above - are effective in imitating migrating stoneflies and those that get dislodged and are vulnerable while in the current. While stoneflies are certainly of less importance in much of Wisconsin, their migration to shore can draw some pretty significant attention from trout.
Adult
Because stonefly nymphs crawl to shore or emergent mid-stream features like rocks or woody debris, there is not the vulnerable transitional stage between being an aquatic larva and an adult that midges, caddis, and mayflies all share. Where stoneflies are most vulnerable are as nymphs when they are dislodged from rocks and during their nymphal migration and as adults when they return to mate and lay eggs. So, unlike mayflies, caddisflies, and midges, there are no emergers or cripples in any number to be imitated. (maybe for the winter stones...)
As adults (imagos), stoneflies are returning to the water to mate, with the females depositing eggs over riffles and rapids. Stoneflies are rather clumsy fliers and often fall less than gracefully on the water. Sometimes a "splat" - like you might do with a grasshopper - helps draw the trouts' attention. Stoneflies are generally fished in two different ways as adults. First, they are active - but clumsy fliers - as they return to mate and lay eggs. These are generally imitated with high floating, heavily hackled flies like stimulators and variations on the theme which can be twitched and actively fished. Foam flies are also commonly fished and they work well in this application but probably are best at imitating the dead and dying stoneflies that dead drift downstream. Any number of "Chernobyl" patterns and derivations of them were designed as all-purpose stonefly and grasshopper imitations.
In Wisconsin, the stonefly adults that we see returning to the water are rarely the large Pteronarcys species but the smaller, daintier winter, early brown, and yellow sally stoneflies. Rather than the large salmonflies of the Western U.S., most of our stoneflies are #14's and smaller. The winter stoneflies are typically #18's as adults and only a bit larger as nymphs. Most of our stoneflies are not the robust species most think of but the slimmer and smaller stonefly species.
Wrapping it Up
Of the major aquatic insect taxa, the EPT taxa - Ephemeroptera - mayflies, Plecoptera - stoneflies, and Trichoptera - caddisflies; stoneflies are typically of the least importance here in Wisconsin. In fact, in Driftless streams I typically fish, crane flies, midges, and scuds (not an insect, I know...) are all of greater importance. On Western streams, stoneflies are "the glory hatches" that anglers from around the World chase - basically our Hex hatch but at a time you can see what the hell you are doing. This is not to say that they can not be important in Wisconsin or even the Driftless Area but they are generally of less importance. However, in the dead of winter, it is nice to see some life above the water and while not long-lived, they can provide a short burst of dry fly activity.
If you ever have a chance to chase the Western stoneflies, you really need to give it a try.
Stonefly Life Cycle Links
Wisconsin Stonefly Hatches
Stonefly Fly Patterns and Videos
And just for fun - tying the Bunyon Bug, Stonefly #2