Stonefly, (order Plecoptera), are any of about 2,000 species of insects, the adults of which have long antennae, weak, chewing mouthparts, and two pairs of membranous wings. The stonefly ranges in size from 6 to more than 60 mm (0.25 to 2.5 inches).
As the name suggests, stoneflies are typically found in rocky and fast flowing rivers and streams, although some species occur on the shores of stony lakes and in ponds and marshes. Adults are more commonly found crawling amongst stones and vegetation on the bankside rather than in flight. They fold their wings flat over their bodies when at rest.
As fisherman, there are just a few important things to know about stoneflies. First, when they are available in numbers, like when ready to "hatch", the fish go nuts! Second, stoneflies are found, almost exclusively in cold fast-running water (highly oxygenated).
The insects remain in the nymphal form for from one to four years, depending on species, and undergo from 12 to 36 molts before emerging and becoming terrestrial as adults. Before becoming adults, nymphs will leave the water, attach to a fixed surface and molt one last time.
The adults generally only survive for a few weeks, and emerge only during specific times of the year when both water and air temperatures are optimal. Some do not feed at all, but those that do are herbivorous. Adults are not strong fliers and generally stay near the stream where they hatched.
For more detailed information...follow these links:
Stoneflies - (credit Troutnut.com)
Flyfisher's Guide to Stoneflies - (video by Skyler Hardman on YouTube
Fishing Stoneflies - (credit: Pat Dorsey article)