This imitation of a mayfly at its most vulnerable stage as it's wriggling from its shuck at the surface- is proven on tough trout in Colorado, Montana, Idaho, Washington, Oregon, and British Columbia. It hangs its nymph-half under the water's surface while its front half (which imitates the thorax now free of the shuck) floats. Skip's found his Morris Emergers as reliable as emerger patterns can be, and longer floating than you'd expect from half-sunk emerger flies- Morris Emergers are stubborn floaters.