BWO or Blue Winged Olives are an important mayfly hatch that happens on almost every river in the world. They are a member of the Baetis family and very common. Typically they are about a size #18 or #16, with #20 sizes being necessary at times. Rarely do you see a true #14 BWO. These nymphs, dry flies, cripples, and emergers are all you need to have some incredible fishing on a Blue Winged Olive Hatch.