Scientific Name: genus Isonychia

Isonychia mayflies are strong swimmers, and live in moderate to fast flows. They are not important everywhere; in fact, they are of no importance whatsoever on most rivers. But where they occur, they are often found in good numbers and can generate aggressive and selective feeding from trout. They hatch during the summer months.

Prior to emergence, the nymphs migrate to slower water. During this migration, many drifting nymphs are taken by trout. So you might try dead-drifting nymph imitations near the bottom beginning a couple of weeks before the hatch starts.

Nymphs crawl out of the water and onto to dry land to emerge, so there is little point in fishing a dun imitation. However, spinner falls can be important.


How to Match a Isonychia

Hatches are matched from Westfly's database of "standard" fly patterns.

Size 8-10 Hares Ear. Dark red-brown
Slow-moderate runs: indicator, tight line
Size 8-10 Rusty Spinner. Dark red-brown
Slow-moderate runs: standard dry fly


NYMPH COLOR: Black, dark red-brown

DUN SIZE: 13-19 mm (1/2 to 3/4 in)

DUN COLOR: Wings--dark gray with mottling. Body--gray, red-brown

SPINNER SIZE: 13-19 mm (1/2 to 3/4 in)

SPINNER COLOR: Wings--clear. Body--dark red-brown or gray

OTHER CHARACTERISTICS: Nymph--slender shape; long hairs on front legs; three fringed tails. Dun and spinner--two tails; large hind wings