March Brown

Telling it apart

• Large: size 10–12, with heavily brown-mottled forewings.
• Mottled brown body, legs banded in dark brown.
• A broad, flattened nymph typical of insects that cling to rock in fast water: a wide head with eyes set far apart on top.
• Emerges in broad daylight, generally between 11 a.m. and 3 p.m. — unusual for a mayfly this size.
• Three tails, like most mayflies.

A fast-water nymph

The nymph's flattened body and widely splayed legs are not decorative — they are the equipment of an insect that lives clinging to rock, right in the current seam, where the water carries the most oxygen. It feeds there by grazing algae and biofilm off the stones.

That dictates the habitat to look for: riffles and runs over clean rock and coarse gravel, not slow silty water. It is almost the opposite of Hexagenia, which needs soft silt to dig its burrow.

An afternoon hatch, a trickle at a time

Unlike a synchronized emergence triggered by falling light, the March Brown's stretches across hours in broad daylight, a few insects at a time. There is almost never a visible wave or a pod of steadily rising fish.

That changes the approach: rather than waiting for a pod of risers, prospect the likely lies with a dry big and buoyant enough to stay visible a long time, and accept that the takes come one at a time, spaced apart.

Two insects, one season

March Brown and Grey Fox share the same water and overlap for nearly six weeks. In practice that means carrying both sizes and following the clock more than the calendar: a brown-mottled 10 or 12 in the afternoon, then a paler 12 or 14 — often a Light Cahill — once the light starts to drop toward evening.