Rim Chung designed the RS2 ("Rim's Semblance #2") in the 1970s for the technical trout of Denver's South Platte. Nothing about the pattern is dramatic — a thread body, a few fibers of synthetic dubbing, a wisp of CDC or Z-Lon for the wing case — but on tailwater fish sipping in the film, it consistently outfishes more elaborate patterns.
Tied in sizes 18-24. Olive and gray are the workhorses; baetis-blue for BWO hatches.
Fish it as a dropper 12-18 inches below a small dry, or as the back fly on a two-emerger team during a midge hatch. The trick is no floatant on the body — only on the wing case — so the fly hangs in the surface tension exactly like the real emerging bug.