The march brown nymph is classified as a clinger nymph, which are flat, wide bodied insects. Clingers are specialists at holding onto the river bottom, even during high flow. Before hatching, the nymph moves to slower water.
Once in slow water, the nymph slowly swims to the surface, where it breaks the surface tension, hatches out of its exoskeleton, rides down the river while drying its wings, and eventually flies off. Emerging mayflies are a favorite of trout because they are defenseless during this part of their lifecycle, and slow to become airborne. A dead drifted emerging mayfly with little or no weight is a perfect imitation, as is a mayfly cripple.