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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.

Case view of a Pycnopsyche guttifera (Limnephilidae) (Great Autumn Brown Sedge) Caddisfly Larva from the Yakima River in Washington
It's only barely visible in one of my pictures, but I confirmed under the microscope that this one has a prosternal horn and the antennae are mid-way between the eyes and front of the head capsule.

I'm calling this one Pycnopsyche, but it's a bit perplexing. It seems to key definitively to at least Couplet 8 of the Key to Genera of Limnephilidae Larvae. That narrows it down to three genera, and the case seems wrong for the other two. The case looks right for Pycnopsyche, and it fits one of the key characteristics: "Abdominal sternum II without chloride epithelium and abdominal segment IX with only single seta on each side of dorsal sclerite." However, the characteristic "metanotal sa1 sclerites not fused, although often contiguous" does not seem to fit well. Those sclerites sure look fused to me, although I can make out a thin groove in the touching halves in the anterior half under the microscope. Perhaps this is a regional variation.

The only species of Pycnopsyche documented in Washington state is Pycnopsyche guttifera, and the colors and markings around the head of this specimen seem to match very well a specimen of that species from Massachusetts on Bugguide. So I am placing it in that species for now.

Whatever species this is, I photographed another specimen of seemingly the same species from the same spot a couple months later.
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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Mayfly Species Eurylophella verisimilis (Chocolate Duns)

Where & when

In 22 records from GBIF, adults of this species have mostly been collected during June (45%), May (32%), and July (18%).

In 19 records from GBIF, this species has been collected at elevations ranging from 20 to 2723 ft, with an average (median) of 620 ft.

Species Range

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Described in Needham et al (1935) as Emphemerella verisimilis
Body length: 7 mm
Wing length: 8 mm

A species of the bicolor group (now a synonym of Eurylophella bicolor), intermediate between E. bicolor (now a synonym of Eurylophella bicolor) and E. temporalis (now a synonym of Eurylophella temporalis); abdominal sternites darker and legs deeper yellow than in these species. Occipital tubercles of nymph well developed; rows of dorsal spines evenly divergent to rearward.

Eyes orange. Head reddish brown with purplish brown shading around the ocelli. Thorax deep brown. Legs rather bright yellow, with traces of orange patches at the base, the middle and the apex of each femur. Wings hyaline, venation hyaline. Abdominal tergites deep brown, with no indication of yellow or orange shading. Abdominal sternites darker than in bicolor or temporalis. Faint indications of the usual dark dots and oblique streaks. Tails smoky, the joinings narrowly darker.


Described in Needham et al (1935) as Ephemerella verisimilis

The occipital tubercles of the nymph are quite well developed in both sexes. The rows of dorsal spines are gradually and evenly divergent to rearward; those of tergites 1-3 are slightly longer and more erect than in E. bicolor, the tips more pointed. The postero-lateral spines on segments 2 and 3 are considerably better developed than in bicolor (now a synonym of Eurylophella bicolor). Tails brown with no pale bands.

Start a Discussion of Eurylophella verisimilis


  • Needham, James G., Jay R. Traver, and Yin-Chi Hsu. 1935. The Biology of Mayflies. Comstock Publishing Company, Inc.

Mayfly Species Eurylophella verisimilis (Chocolate Duns)

Species Range
Common Name
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