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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.
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Dark Olive Duns

Like most common names,"Dark Olive Dun" can refer to more than one taxon. They're previewed below, along with 3 specimens. For more detail click through to the scientific names.

Mayfly Species Drunella coloradensis

These are very rarely called Dark Olive Duns.
This species is very similar to Drunella flavilinea. In areas where their ranges overlap, they can sometimes be found in the same streams. They are similar enough that anglers sometimes refer to either or both species as "Flavs." Allen and Edmunds (1962) say that Drunella coloradensis tends to favor colder water than Drunella flavilinea and that it may emerge as much as a month later.
Lateral view of a Male Drunella coloradensis (Ephemerellidae) (Small Western Green Drake) Mayfly Dun from Mystery Creek #199 in Washington
The positive species ID on this dun comes from both the spinner that it (or possibly one other dun just like it) molted into and the overwhelming abundance of nymphs of this species in my kicknet samples from the same site.
Lateral view of a Male Drunella coloradensis (Ephemerellidae) (Small Western Green Drake) Mayfly Spinner from Mystery Creek #199 in Washington
This spinner molted from this dun, or possibly one other dun I had in the same container that looked just like it.
Dorsal view of a Drunella coloradensis (Ephemerellidae) (Small Western Green Drake) Mayfly Nymph from Mystery Creek #249 in Washington
This one nicely illustrates the variation in coloration within an single Ephemerellid species in a single stream, when compared to its lighter, banded counterpart.

Mayfly Species Siphlonurus rapidus

These are very rarely called Dark Olive Duns.
This species may reinforce spinner flights of Siphlonurus quebecensis and Siphlonurus alternatus.

Dark Olive Duns

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