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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.

Dorsal view of a Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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.

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

South Carolina

Posts: 1
KGotz on May 31, 2008May 31st, 2008, 4:36 am EDT
I have what I believe to be these Mayflies all over my house. They swarm and fly into you. I have found hundreds of exoskeletons all over my window screens. I do not want them here. When will they leave....

Please help. I have pictures if anyone can help me determine if this is the right insect.

Kim Gotz
South Carolina
Troutnut's profile picture
Bellevue, WA

Posts: 2758
Troutnut on May 31, 2008May 31st, 2008, 8:57 am EDT
They probably are mayflies. Each species is usually only around for a week or so, so they'll go away on their own (until the same time next year). If you're right near a river and you just moved in, this might be a regular issue you've got, with a variety of different species. If so, the only thing I can recommend is not leaving many lights on at night. (They fly to your house because they're attracted by the lights at night.) Of course, you could always sell your house to somebody who likes having lots of mayflies around, too...
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist
SUNY Environmental Science and Forestry, Syracuse

Posts: 71
Jjlyon01 on May 31, 2008May 31st, 2008, 11:07 am EDT
Choose me I'll take it
"I now walk into the wild"
CaseyP's profile picture
Arlington, VA/ Mercersburg, PA

Posts: 653
CaseyP on Jun 1, 2008June 1st, 2008, 3:53 am EDT
you could always sell your house to somebody who likes having lots of mayflies around

or here's a deal: when you figure out which week(s) that the mayflies are there, make a house swap with one of us and we'll all come down and look after your lovely home, and you can enjoy Our Nation's Capital, or beautiful central Pennsylvania, or an equally lovely part of New York, or places further west. we're from all over around here--you could take your pick, i'm sure!
"You can observe a lot by watching." Yogi Berra
Posts: 1
Ffhd1clt on Jun 27, 2008June 27th, 2008, 4:12 am EDT
I live on Lake Norman outside of Charlotte, and the Mayflies have hit today. Huge swarms of them are flying in my backyard, and they are thick on my house nad on the treees and shrubs. They can be quite annoying, but here's the good news: They only come around once a year, and only stay around about a week (most of them only live about a day as adults). They are harmless to humans and our pets. And finally, if you have Mayflies around, you know that your lake or stream is healthy. They can't live in polluted water, which is important because they are born in the water.

Lake Norman, NC

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