Most stoneflies take two or three years to mature, and it makes sense that there are always nymphs around that are two-thirds of their adult size. Even during hatches. The most effective nymphs are weighted. Unlike mayflies and caddis that enter this world through the surface of the stream, stoneflies crawl out to the sides to hatch. They don't swim, staying at or near the bottom, moving around freely, and becoming more active as they mature.