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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 Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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 September 3, 2020

Updates from September 3, 2020

Photos by Troutnut from Silver Creek and Trail Creek in Idaho

Silver Creek in Idaho
Silver Creek in Idaho
Silver Creek in Idaho
Silver Creek in Idaho
Silver Creek in Idaho
Silver Creek in Idaho
Silver Creek in Idaho
Trail Creek in Idaho
Trail Creek in Idaho
Trail Creek in Idaho

On-stream insect photos by Troutnut from Silver Creek in Idaho

Silver Creek in Idaho
Female Baetid spinners were abundant subsurface looking for substrates to lay their eggs, and they found my waders. I'll update the ID for this picture after identifying the associated male spinner. I caught several fish on a small brown soft-hackle fished as a crude imitation of these spinners before the fish all started taking Tricorythodes spinners instead.

From Silver Creek in Idaho

Closeup insects by Troutnut from Silver Creek in Idaho

Artistic view of a Female Callibaetis ferrugineus (Baetidae) (Speckled Dun) Mayfly Spinner from Silver Creek in Idaho
Male Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Spinner from Silver Creek in Idaho
Some notes from identifying this specimen under the microscope:

1. The hind wing has three longitudinal veins, but the third is faint, short (about half the length of the wing), and close to the wing margin.
2. Then antenna is brown fading into white at the tip, and the base is ringed with white.
3. The joints of the tarsal segments on the middle and hind leg have fine black markings.

It was also collected in association with a female spinner.
Lateral view of a Female Baetis tricaudatus (Baetidae) (Blue-Winged Olive) Mayfly Spinner from Silver Creek in Idaho
This female spinner was deceased when photographed, so the posture is unnatural, but it was a substantial hatch and clearly worth imitating (with wet flies to imitate the sunken ovipositing females) so I went ahead with the photos.

It was collected in association with a male spinner.

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Jun 28, 2007
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