Isonychia (Iso)

Overview The Isonychia, commonly known as the "ISO" or Leadwing Coachman, Slate Drake, or Mahogany Dun, is a large and robust mayfly cherished by anglers for its consistent appearance throughout the fishing season. Found in riffles and fast-moving sections of streams and rivers, this mayfly is a favorite target for trout due to its substantial size and enticing movement. Its staggered hatches often extend over months, providing anglers with ample opportunity to fish both nymphs and dries successfully.

Nymph Stage Nymphs of Isonychia are slender and elongated, typically measuring around 0.5 to 1 inch in length. They are dark brown to mahogany in color, with pale or cream-colored accents on their undersides and legs. These nymphs are strong swimmers and are often found clinging to rocks in fast currents. When imitating this stage, anglers typically use hook sizes 10–14. Their active swimming motion makes them ideal for fishing with a dead-drift or subtle twitch retrieval.

Adult/Dry/Dun Stage The adult Isonychia mayfly ranges from 0.75 to 1 inch in length, with dark slate-gray wings and a rich mahogany or dark reddish-brown body. They are most active during late afternoons and evenings, particularly in the warmer months. Imitations of the dry fly are typically tied on hook sizes 10–14 to match their impressive size. Anglers should focus on slower water near riffles or eddies, where adults often gather during emergence or egg-laying. Their distinctive fluttering flight pattern can help anglers identify their presence on the water.