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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

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.
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Mayfly Family Isonychiidae (Slate Drakes)

See Isonychia for details. It is the only North American genus in this family.

Where & when

In 120 records from GBIF, adults of this family have mostly been collected during June (38%), July (31%), August (16%), and September (6%).

In 42 records from GBIF, this family has been collected at elevations ranging from 180 to 5407 ft, with an average (median) of 3018 ft.

Family Range


To determine whether a specimen of Ephemeroptera belongs to Isonychiidae, use the Key to Families of Mayfly Nymphs or Key to Families of Mayfly Duns and Spinners.

Specimens of the Mayfly Family Isonychiidae

5 Female Duns
1 Male Spinner
2 Female Spinners
13 Nymphs

5 Streamside Pictures of Isonychiidae Mayflies:

Discussions of Isonychiidae

Isonichya Bicolor
21 replies
Posted by CraigK on Mar 26, 2012
Last reply on Mar 29, 2012 by Entoman
I think the Iso. b. female was referred to a generation or so ago as the white gloved howdy. I love those old names. Too bad there are no pics of Potamanthus (golden drake). They may be extinct (siltation and acid rain?)...talked with Charlie Meck about that a few years ago. A beautiful mayfly...an important hatch of years past. I couldn't find any ref. to Epherons. An important hatch for me in about any area of the country. This my first post...love the site...very nice photog. Lets see what the strange weather of the year does to the hatches and fishing for this year. Overall, I expect it can't be good. CK
Are Isonychia mayflies technically multibrooded?
4 replies
Posted by Troutnut on Jul 22, 2006 in the genus Isonychia
Last reply on Apr 17, 2009 by GONZO
Here's what I've written in my article on Isonychia about their hatching:

Some Isonychia species are multibrooded, but not in the same way as most other multibrooded mayflies like the Baetidae. In those species, the flies emerging in midsummer or Fall are the offspring of the earlier hatch from the same year. In Isonychia, the Fall emergers are offspring from the previous Fall. They are present as half-grown nymphs when the first of their generation emerge. Although Isonychia broods have distinct peaks, some may be found on the water at any time in between.

I'm curious if they can really be called multibrooded or not, since they don't produce more than one generation per year (as far as I know). They simply have distinct populations within the same generation which emerge at different times during the year. Does that count?

All my books are packed up in boxes right now so I don't have a technical definition of the term handy.
Penns Creek Slate Draker's
4 replies
Posted by Jsell925 on Jul 17, 2007 in the genus Isonychia
Last reply on Sep 23, 2007 by Shawnny3
Penns is one of the few places where a #10 iso will nail em' all year long
1 replies
Posted by JMV on Sep 21, 2006 in the species Isonychia bicolor
Last reply on Sep 21, 2006 by Troutnut
Great site, I'm an Iso. fanatic... JM

Start a Discussion of Isonychiidae

Mayfly Family Isonychiidae (Slate Drakes)

Genus in Isonychiidae
Genus in Isonychiidae: Isonychia
Family Range
Common Name
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