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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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This topic is about the Insect Order Ephemeroptera

Mayflies may be the most important insects for trout anglers to understand. They are an ancient order of insects, famous outside the fly-fishing world for their fragile beauty and short adult lifespan, often a single day to mate and die. The mayfly's poignant drama attracts poets and anglers alike, but anglers make the most of it.

Mayflies live more than 99% of their lives as nymphs on the river or lake bottom, filling many crucial roles in freshwater ecosystems as they feed and grow. They eventually emerge from the water as winged sub-adults called "subimagos" by scientists and "duns" by anglers. Duns evolved to be good at escaping the water, with a hydrophobic surface and hardy build, but they are clumsy fliers. Within a day or two they molt one last time into "imagos" or "spinners," the mature adults, a transformation captured in this photo series of a dun molting into a spinner. They have longer legs and tails, and sleeker, more lightweight bodies, giving them the airborne speed, agility, and long grasp they need for their midair mating rituals. They are usually darker than the duns and have shinier, more transparent wings. They die within minutes or hours after mating.

Example specimens

Posts: 3
Deligon on Sep 8, 2008September 8th, 2008, 3:42 pm EDT
I have had using a fly a friend turned me on to ,Isonychia parachute.
If iso nymphs hatch into duns on rockswood is the dry fly (parachute)
the spinner stage? all I know is it works,
I have fished my whole life but this only my second year fly fishing.
I fish the Hiwassee and Elk in Tennessee.
The more I learn the more I discover I don't know.

Site Editor
"Bear Swamp," PA

Posts: 1681
GONZO on Sep 9, 2008September 9th, 2008, 7:55 am EDT

Although the appearance of their dark nymphal shucks on streamside rocks is often a familiar sign of an Isonychia emergence, they have the ability to emerge in the water as well. Conditions may dictate that one or the other method might predominate, but I doubt that either method is employed exclusively. Many anglers find that imitations of the duns are attractive to trout, even at times when most emerge on streamside rocks. This may be due to their lengthy emergence, often consisting of multiple groups or "cohorts." In streams with good populations, the trout seem very familiar with Isonychia and readily take the imitation of all stages.

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