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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 Glossosoma (Glossosomatidae) (Little Brown Short-horned Sedge) Caddisfly Larva from the Yakima River in Washington
I caught this tiny larva without a case, but it seems to key pretty clearly to to Glossosomatidae. From there, the lack of sclerites on the mesonotum points to either Glossosoma or Anagapetus. Although it's difficult to see in a 2D image from the microscope, it's pretty clear in the live 3D view that the pronotum is only excised about 1/3 of its length to accommodate the forecoxa, not 2/3, which points to Glossosoma at Couplet 5 of the Key to Genera of Glossosomatidae Larvae.
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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Updates from October 15, 2011

Updates from October 15, 2011

Closeup insects by Entoman from the Fall River in California

Female Ephemerella excrucians (Ephemerellidae) (Pale Morning Dun) Mayfly Dun from the Fall River in California
Size: 10mm. At emergence the specimen was a fairly bright olive green and there was obvious difference in color between the forewing (med. dun) and the hind-wing (pale cream). It was really noticeable as they floated by. You can just make these features out in the second photo, but not so much in the first that was taken 24 hours after capture. Total time from emergence to molting - approx. 48 hours.

Entoman


Edit 2/25/13 - This specimen was originally posted to E. d. infrequens because of its size. It turns out large size doesn't hold up as a way to tell these two apart. This is because excrucians has much greater variability than previously understood. The assumption by anglers that excrucians is always the smaller of the two is apparently not supported by science. There is a lot left to sort out with western Ephemerella species. This may include new discoveries and/or synonyms as well as reportage on new intraspecific variations broadening the descriptions of recognized species. Based on this specimen's Fall maturity, the best guess is that it is an unusual form of excrucians.

As to color, both species duns (nymphs too) demonstrate a tremendous amount of intraspecific variability from pale yellow to bright green with a multitude of sulfur shadings in between, ranging from pale amber, through orange to cinnamon and even dark brown. I've seen wings from pale cream through tannish and almost every shade of dun except the dark shades. Some have pigment stained leading edges matching their bodies, some don't. Most of these variations are undocumented except in angler references and periodicals. It seems a rare year that a new variation doesn't pop up to the notice of anglers.

Bottom line - size is only reliable if the specimens are smaller than size 16, pointing to excrucians. Otherwise, the only fairly dependable way to tell them apart (especially the females) is by timing as infrequens is the first of the two to appear, rarely lasting longer than a couple of weeks or later than the end of June most years. The problem with using timing for determination is it requires knowledge of the hatch sequences as they actually occurred for a given year on a given piece of water. Obviously, this kind of information is seldom available. Without it, determining between the two duns if they are larger than size 18 is speculative at best - at least until very late in the Summer.

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