The Blue Wing Olive (BWO) fly is a popular and effective pattern in fly fishing. It is designed to imitate the olive-colored mayflies, particularly during their hatching stage. The fly typically features a slender body made with olive or gray dubbing, a set of upright or trailing wings to mimic the mayfly's distinct wing profile, and a hackle collar for added buoyancy. Blue Wing Olive flies are commonly used when these mayflies are present on the water, making them a go-to pattern for trout anglers.