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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives
Baetis

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Lateral view of a Onocosmoecus (Limnephilidae) (Great Late-Summer Sedge) Caddisfly Larva from the Yakima River in Washington
This specimen keys pretty easily to Onocosmoecus, and it closely resembles a specimen from Alaska which caddis expert Dave Ruiter recognized as this genus. As with that specimen, the only species in the genus documented in this area is Onocosmoecus unicolor, but Dave suggested for that specimen that there might be multiple not-yet-distinguished species under the unicolor umbrella and it would be best to stick with the genus-level ID. I'm doing the same for this one.
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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Yellow Mays

Like most common names,"Yellow May" can refer to more than one taxon. They're previewed below, along with 9 specimens. For more detail click through to the scientific names.

Mayfly Species Stenacron interpunctatum

These are very rarely called Yellow Mays.
This is the best of the sporadic summer hatches known to anglers as Light Cahills. It was formerly known as Stenonema canadense and is considered by most angling historians as the model for that classic Catskill pattern.
Lateral view of a Female Stenacron interpunctatum (Heptageniidae) (Light Cahill) Mayfly Dun from the West Branch of Owego Creek in New York
This is a fairly small late-season female Stenacron dun.
Lateral view of a Male Stenacron interpunctatum (Heptageniidae) (Light Cahill) Mayfly Spinner from the Namekagon River in Wisconsin
Dorsal view of a Stenacron interpunctatum (Heptageniidae) (Light Cahill) Mayfly Nymph from the Marengo River in Wisconsin

Mayfly Species Ephemerella invaria

These are very rarely called Yellow Mays.
This species, the primary "Sulphur" hatch, stirs many feelings in the angler. There is nostalgia for days when everything clicked and large, selective trout were brought to hand. There is the bewildering memory of towering clouds of spinners which promise great fishing and then vanish back into the aspens as night falls. There is frustration from the maddening selectivity with which trout approach the emerging duns--a vexing challenge that, for some of us, is the source of our excitement when Sulphur time rolls around.

Ephemerella invaria is one of the two species frequently known as Sulphurs (the other is Ephemerella dorothea). There used to be a third, Ephemerella rotunda, but entomologists recently discovered that invaria and rotunda are a single species with an incredible range of individual variation. This variation and the similarity to the also variable dorothea make telling them apart exceptionally tricky.

As the combination of two already prolific species, this has become the most abundant of all mayfly species in Eastern and Midwestern trout streams.
Lateral view of a Male Ephemerella invaria (Ephemerellidae) (Sulphur) Mayfly Dun from Penn's Creek in Pennsylvania
Lateral view of a Male Ephemerella invaria (Ephemerellidae) (Sulphur) Mayfly Spinner from the Teal River in Wisconsin
Dorsal view of a Ephemerella invaria (Ephemerellidae) (Sulphur) Mayfly Nymph from the Beaverkill River in New York
This small Ephemerella invaria nymph was at least a month away from emergence.

Mayfly Species Ephemera varia

These are very rarely called Yellow Mays.
This is an excellent hatch of a different character than its Ephemera brethren. Rather than emerging in a flurry of activity within a week, the Ephemera varia hatch may last for more than a month in a single place.
Artistic view of a Female Ephemera varia (Ephemeridae) (Yellow Drake) Mayfly Dun from Aquarium in New York
This yellow drake dun hatched out of my aquarium over a month before her brethren in the wild are slated to emerge. She seems a bit small, and that might be the reason.
Lateral view of a Female Ephemera varia (Ephemeridae) (Yellow Drake) Mayfly Spinner from Cayuta Creek in New York
I found this female spinner ovipositing in a small stream. She came along while I was playing a trout -- every good bug seemed to do that last night! I didn't have my bug net, so I caught the trout in my landing net, released the trout, and caught the mayfly in my landing net. Her wing got a bit messed up from that.
A burrowing mayfly nymph. The juvenile stage of the brown drake, Ephemera simulans

Dorsal view of a Ephemera varia (Ephemeridae) (Yellow Drake) Mayfly Nymph from Fall Creek in New York
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