Header image
Enter a name
Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Species Isonychia rufa (Slate Drakes)

Where & when

In 48 records from GBIF, adults of this species have mostly been collected during July (40%), June (31%), August (19%), and September (6%).

In 29 records from GBIF, this species has been collected at elevations ranging from 781 to 5407 ft, with an average (median) of 3150 ft.

Species Range

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Body length: 10 mm
Wing length: 10 mm

A rather bright red species, with pale venation. Eyes ruddy chestnut in living male. Head light brown, bases of the ocelli black-ringed. Thorax ruddy brown. Coxae and femora of fore legs deep reddish; tibiae blackish; tarsi dull yellowish white, the apical portion of each joint shaded with smoky. The tarsus is as long as the tibia; the whole leg is somewhat longer than in Isonychia sicca. Middle and hind legs rather deep yellowish. A small black lateral spot on the occiput. Wings hyaline, with pale venation in the male.

Abdomen bright reddish dorsally, paler and duller ventrally. No blackish shading except a narrow posterior margin on each segment. Tails whitish yellow; traces of reddish rings in the basal joinings. Forceps pale reddish brown. Forceps base deeply excavated on its apical margin. Lateral arms of the forceps base rather broad, the inner margins wide and flaring, somewhat angulate apically. Penes of the albomanicata type (now a synonym of Isonychia bicolor).

Female Spinner

Body length: 10 mm
Wing length: 10 mm

Head of female pale yellow, shaded with brown around the bases of the ocelli and between the eyes and the lateral ocelli.
Wings hyaline; in the female, the cross veins may be faintly tinged with brown. Subanal plate of the female deeply excavated apically, its outer angles acute.

Start a Discussion of Isonychia rufa


  • Needham, James G., Jay R. Traver, and Yin-Chi Hsu. 1935. The Biology of Mayflies. Comstock Publishing Company, Inc.

Mayfly Species Isonychia rufa (Slate Drakes)

Species Range
Troutnut.com is copyright © 2004-2024 (email Jason). privacy policy