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Artistic view of a Male Pteronarcys californica (Pteronarcyidae) (Giant Salmonfly) Stonefly Adult from the Gallatin River in Montana
Pteronarcys californica

The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.

Dorsal view of a Skwala (Perlodidae) (Large Springfly) Stonefly Nymph from the Yakima River in Washington
This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
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: 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's profile picture
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's profile picture
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.
"old habits are hard to kill once you have gray in your beard" -Old Red Barn
Konchu's profile picture
Site Editor

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.
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's profile picture
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.
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.


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