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.

Stream photos

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Coleoptera (Beetle) Insect Larva Coleoptera (Beetle) Insect Larva

Lateral view of a Coleoptera (Beetle) Insect Larva from Sears Creek in Washington

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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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swarm sighting along Ohio River in Pittsburgh

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

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Male Epeorus albertae (Pink Lady) Mayfly Spinner

Lateral view of a Male Epeorus albertae (Heptageniidae) (Pink Lady) Mayfly Spinner from the North Fork Stillaguamish River in Washington

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Male Epeorus deceptivus Mayfly Dun

Artistic view of a Male Epeorus deceptivus (Heptageniidae) Mayfly Dun from the South Fork Sauk River in Washington

The lack of a darkened humeral crossvein rules out Epeorus albertae and Epeorus dulciana. The lack of a dark macula on the forefemora rules out Epeorus longimanus. The small size rules out Epeorus grandis and Epeorus permagnus. That leaves as the only possibility known in Washington state Epeorus deceptivus. It is a small species, although not reportedly quite as small as this specimen. I couldn't find anything in the species description in Traver (1935) to definitively confirm or rule out the species ID, given that I don't have the preserved specimen to check under a microscope, but it has to be either deceptivus or something not yet reported in Washington. It was collected at the same time as a similar-sized female dun.

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Female Acentrella turbida (Tiny Blue-Winged Olive) Mayfly Spinner

Lateral view of a Female Acentrella turbida (Baetidae) (Tiny Blue-Winged Olive) Mayfly Spinner from the Middle Fork Snoqualmie River in Washington

Cgrier
Posts: 1

Cgrier on Jun 24, 2007June 24th, 2007, 1:32 pm EDT

I witnessed what I think was a mayfly swarm along Ohio River Blvd in Pittsburgh on Thursday, June 21st at dusk. The insects created spinning columns that were about 30-80 feet high. There were about 150-200 of these columns for about 1/2 mile along the hillside just above the sewage treatment plant, and also above a stream that feeds into the Ohio. I have been searching but can't find any documentation that the swarming behavior occurs in this column pattern. Is anyone familiar with this?

Martinlf

Moderator
Palmyra PA

Posts: 3047

Martinlf on Jun 25, 2007June 25th, 2007, 4:50 am EDT

It sounds like mating spinners to me.

"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell

JOHNW

Chambersburg, PA

Posts: 452

JOHNW on Jun 26, 2007June 26th, 2007, 10:34 am EDT

I'll second Louis on the mating swarms.
We get very similar occurances on the Susquehanna.
Out of curiosity how big were these bugs? Are we talking Hummingbird or smaller?
I know the Pittsburgh area sees some pretty dramatic Hexegenia hatches. They sort of bring Alfred Hitchcok's BIRDS to mind and I'm sure to head the other way.
JW

"old habits are hard to kill once you have gray in your beard" -Old Red Barn

Konchu

Site Editor
Indiana

Posts: 498

Konchu on Jul 1, 2007July 1st, 2007, 7:19 am EDT

This thread reminded me of an old report of a mayfly swarm that I thought some might enjoy.

"...a great swarm of ephemerids...passed through the town of Lewisburg, on the Susquehanna River, on the afternoon of the 22nd of August. The swarm was estimated to be about a mile in length by nearly a half mile in width, and was so dense as even to obscure passers by on the opposite side of the street."

Gentry, TG. 1873. Proceedings of the Academy of Natural Sciences of Philadelphia, volume 25, 3rd series, page 350.

Aaaa
Posts: 1

Aaaa on Sep 26, 2007September 26th, 2007, 4:33 am EDT

Just wanted to add that I saw a series of insect swarms a couple of days ago (late Sept). These were also tall spinning columns, near water in a line above a row of trees paralleling a highway. Also at dusk, temp about 80, and also just across the highway from a sewage treatment plant. I'd have guessed about 30 columns in view, each around 40 feet high. I didn't get close enough to tell what the insect might be, but bigger than a housefly. This is in Washington DC.

Wbranch

York & Starlight PA

Posts: 2635

Wbranch on Mar 14, 2008March 14th, 2008, 3:40 pm EDT

Here is a picture of a little swarm.

Catskill fly fisher for fifty-five years.

Creno
Grants Pass, OR

Posts: 302

Creno on Mar 15, 2008March 15th, 2008, 3:56 am EDT

Folks from the east may already know of this but here is a link to an article about a big swarm.

http://seagrant.psu.edu/publications/fs/Mayfly_12-2003.pdf