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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Female Sweltsa borealis (Boreal Sallfly) Stonefly Adult Female Sweltsa borealis (Boreal Sallfly) Stonefly Adult

Lateral view of a Female Sweltsa borealis (Chloroperlidae) (Boreal Sallfly) Stonefly Adult from Harris Creek in Washington

I was not fishing, but happened to be at an unrelated social event on a hill above this tiny creek (which I never even saw) when this stonefly flew by me. I assume it came from there. Some key characteristics are tricky to follow, but process of elimination ultimately led me to Sweltsa borealis. It is reassuringly similar to this specimen posted by Bob Newell years ago. It is also so strikingly similar to this nymph from the same river system that I'm comfortable identifying that nymph from this adult. I was especially pleased with the closeup photo of four mites parasitizing this one.

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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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Caddisfly Genus Phryganea (Rush Sedges)

Trout may feed heavily on any stage of these large caddisflies.

Where & when

In 115 records from GBIF, adults of this genus have mostly been collected during August (35%), July (35%), June (20%), and September (7%).

In 67 records from GBIF, this genus has been collected at elevations ranging from 10 to 10157 ft, with an average (median) of 1027 ft.

Genus Range

source

Map of specific collection sites

Hatching behavior

The pupae crawl out onto shore to emerge, but it may be useful to imitate their subsurface movements.

Egg-Laying behavior

Time of day: Evening

The large adults have very unusual egg-laying behavior, described well by Gary LaFontaine in Caddisflies:

In the evening they begin the mating and egg-laying activities, the females flying thirty to forty feet up in the air and dive-bombing the surface. The large insects hit the water with such a force that they send up small splashes. During intense egg-laying activity they may make a smooth surface look rain splattered.

Swisher and Richards describe a different behavior for the genus in Selective Trout:

Females lay their eggs on the water's surface and run across the water to return to shore.

Readers familiar with this genus are encouraged to reconcile these conflicting accounts in the comments. It may be that both are correct, and the running behavior follows after the splash-down.