Slate Drake (Isonychia)

Telling it apart

• Slate-grey to reddish-brown body, with a distinctive pale stripe under the thorax.
• A fast, muscular swimmer, size 10–12, three long tails.
• Nymph found in fast current, clinging to rock between bursts of swimming — never in dead water.
• Crawls out onto exposed rock to emerge, like the Grey Drake but in fast water rather than calm.

A fast-water swimmer

Unlike the Grey Drake, a swimmer of bays and calm water, Isonychia lives in fast current — alternating between clinging to rock and short bursts of powerful swimming to hold position. It's one of the few mayfly nymphs able to hold its ground in current most other species couldn't face head-on.

Look for it in steady-current seams and deep riffles, never in the dead water where its behavioural cousin lives.

The same out-of-water exit, in fast water

Like the Grey Drake, the Isonychia nymph crawls out of the water to emerge rather than doing so at the surface — but it does so on exposed rock in the riffles rather than on calm-water vegetation. Same behaviour, adapted to a completely different habitat.

That means, as with the Grey Drake, the emergence itself is rarely seen — but unlike it, the Isonychia nymph fishes very well during its migration toward shore, on a tight drift near bottom in fast current.

Two broods, two seasons

Isonychia runs two broods a year: one in early summer, a second that stretches from August into October and carries the very tail end of the dry-fly season. That second brood is especially valuable — on a cool fall day when almost nothing else is hatching, it's often the only big fly still active.

A wet fly fished on a tight drift, left to swing at the end of the presentation, imitates well a nymph actively swimming toward shore before it emerges — and it works especially well during that late-season final push.