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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 Kogotus (Perlodidae) Stonefly Nymph from Mystery Creek #199 in Washington
This one pretty clearly keys to Kogotus, but it also looks fairly different from specimens I caught in the same creek about a month later in the year. With only one species of the genus known in Washington, I'm not sure about the answer to this ID.
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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Ephemerella mucronata Mayfly Nymph Pictures

This is an interesting one. Following the keys in Merritt R.W., Cummins, K.W., and Berg, M.B. (2019) and Jacobus et al. (2014), it keys clearly to Ephemerella. Jacobus et al provide a key to species, but some of the characteristics are tricky to interpret without illustrations. If I didn't make any mistakes, this one keys to Ephemerella mucronata, which has not previously been reported any closer to here than Montana and Alberta. The main character seems to fit well: "Abdominal terga with prominent, paired, subparallel, spiculate ridges." Several illustrations or descriptions of this holarctic species from the US and Europe seem to match, including the body length, tarsal claws and denticles, labial palp, and gill shapes. These sources include including Richard Allen's original description of this species in North America under the now-defunct name E. moffatae in Allen RK (1977) and the figures in this description of the species in Italy.

Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Dorsal view of a Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ventral view of a Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ruler view of a Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington The smallest ruler marks are 1 mm.
Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
The contrast in this image is artificially enhanced to show the fine setae toward the middle-outer end of the tails.

Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Closeup of the hind tarsal claw.

Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Contrast was enhanced in this image to show the intrasegmental setae on the tails.

Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
The ventral lamella of the gill on abdominal segment 6 is boxed in red. The dorsal lamella was peeled back with a pin.

Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ventral view of the right maxilla

Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Dorsal view of the right maxilla

Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington

This mayfly was collected from the Yakima River in Washington on February 18th, 2023 and added to Troutnut.com by Troutnut on February 21st, 2023.


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Ephemerella mucronata Mayfly Nymph Pictures

Collection details
Location: Yakima River, Washington
Date: February 18th, 2023
Added to site: February 21st, 2023
Author: Troutnut
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