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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.

Dorsal view of a Holocentropus (Polycentropodidae) Caddisfly Larva from the Yakima River in Washington
This one seems to tentatively key to Holocentropus, although I can't make out the anal spines in Couplet 7 of the Key to Genera of Polycentropodidae Larvae nor the dark bands in Couplet 4 of the Key to Genera of Polycentropodidae Larvae, making me wonder if I went wrong somewhere in keying it out. I don't see where that could have happened, though. It might also be that it's a very immature larva and doesn't possess all the identifying characteristics in the key yet. If Holocentropus is correct, then Holocentropus flavus and Holocentropus interruptus are the two likely possibilities based on range, but I was not able to find a description of their 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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American March Browns

Like most common names,"American March Brown" can refer to more than one taxon. They're previewed below, along with 5 specimens. For more detail click through to the scientific names.

Mayfly Species Stenonema vicarium

These are often called American March Browns.
In the East and Midwest this is one of the most important hatches of the Spring. They are large flies which emerge sporadically, making for long days of good fishing.

This species contains the two classic Eastern hatches formerly known as Stenonema vicarium and Stenonema fuscum, the "March Brown" and "Gray Fox." Entomologists have discovered that these mayflies belong to the same species, but they still display differences in appearance which the trout notice easily. Anglers should be prepared to imitate both types.
Artistic view of a Male Stenonema vicarium (Heptageniidae) (March Brown) Mayfly Dun from the Namekagon River in Wisconsin
I collected this mayfly on the same trip as a female of the same species. After these photos it molted into a spinner. This is the form of Stenonema vicarium which anglers call the "Gray Fox."
Lateral view of a Female Stenonema vicarium (Heptageniidae) (March Brown) Mayfly Spinner from the Namekagon River in Wisconsin
I collected this mayfly on the same trip as a male of the same species. They are Maccaffertium vicarium mayflies of the type formerly known as Stenonema fuscom, the "Gray Fox."
Dorsal view of a Stenonema vicarium (Heptageniidae) (March Brown) Mayfly Nymph from the Beaverkill River in New York

Mayfly Species Litobrancha recurvata

These are very rarely called American March Browns.
Litobrancha recurvata is generally reported to be the largest North American species of mayfly in angler entomologies, though this understanding is being challenged by reports of Hexagenia limbata that may exceed 40mm in some locales. Regardless, it is certainly the largest mayfly in the region of its distribution. Sometimes it appears together with species of Hexagenia or Ephemera, but in other places it creates excellent action on its own.
Female Litobrancha recurvata (Ephemeridae) (Dark Green Drake) Mayfly Dun from the Au Sable River (Mainstream) in Michigan
These photos were contributed by Spencer Vanderhoof.
Male Litobrancha recurvata (Ephemeridae) (Dark Green Drake) Mayfly Spinner from the Au Sable River (Mainstream) in Michigan
These photos were contributed by Spencer Vanderhoof.

American March Browns

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