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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 Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
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.
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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Small Western Green Drakes

Like most common names,"Small Western Green Drake" 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 Drunella coloradensis

These are often called Small Western Green Drakes.
This species is very similar to Drunella flavilinea. In areas where their ranges overlap, they can sometimes be found in the same streams. They are similar enough that anglers sometimes refer to either or both species as "Flavs." Allen and Edmunds (1962) say that Drunella coloradensis tends to favor colder water than Drunella flavilinea and that it may emerge as much as a month later.
Lateral view of a Male Drunella coloradensis (Ephemerellidae) (Small Western Green Drake) Mayfly Dun from Mystery Creek #199 in Washington
The positive species ID on this dun comes from both the spinner that it (or possibly one other dun just like it) molted into and the overwhelming abundance of nymphs of this species in my kicknet samples from the same site.
Lateral view of a Male Drunella coloradensis (Ephemerellidae) (Small Western Green Drake) Mayfly Spinner from Mystery Creek #199 in Washington
This spinner molted from this dun, or possibly one other dun I had in the same container that looked just like it.
Dorsal view of a Drunella coloradensis (Ephemerellidae) (Small Western Green Drake) Mayfly Nymph from Mystery Creek #249 in Washington
This one nicely illustrates the variation in coloration within an single Ephemerellid species in a single stream, when compared to its lighter, banded counterpart.

Mayfly Species Drunella flavilinea

These are often called Small Western Green Drakes.
The Flavs pick up about a week after the closely related but larger Western Green Drakes (Drunella grandis and Drunella doddsii) finish hatching on most Western waters.

Their hatches may be complemented by simultaneous hatches of two less prolific species, Drunella coloradensis and Drunella spinifera.
Artistic view of a Male Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Dun from the Cedar River in Washington
This dun is smaller than would normally be expected for Drunella flavilinea, but it seems to fit the physical description of that species fairly well. I wasn't sure from the dun pictures alone, so I was hoping it would molt into a spinner. It never made it out of the dun's shuck, but it did begin the process, which allowed me with great care to tease out the last couple abdominal segments of the spinner from the still tightly attached dun shuck, enabling a more confident ID based on the shape of the spinner's penes and sub-genital plate. I also collected a nymph of flavilinea in the same river on the same evening, as well as a shed exuvium that looked large enough to belong to an emerging dun.

The other possibility I considered for a while based on the dun was Drunella pelosa, which would be the right size, but the other characteristics don't fit.
Male Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Spinner from the Flathead River-lower in Montana
Dorsal view of a Drunella flavilinea (Ephemerellidae) (Flav) Mayfly Nymph from the Cedar River in Washington

Mayfly Species Drunella spinifera

These are very rarely called Small Western Green Drakes.
This taxon prefers cold water and does not flourish where water temperatures exceed 60 degrees. As with the slightly larger Drunella coloradensis, this species prefers cold tailwaters, high elevation headwaters and spring creeks.
Female Drunella spinifera (Ephemerellidae) (Western Slate Olive Dun) Mayfly Dun from Fern Creek in Montana
Male Drunella spinifera (Ephemerellidae) (Western Slate Olive Dun) Mayfly Spinner from Fern Creek in Montana
Lateral view of a Male Drunella spinifera (Ephemerellidae) (Western Slate Olive Dun) Mayfly Nymph from Mystery Creek #199 in Washington
In a bucket full of Drunella coloradensis nymphs, this was the only specimen of Drunella spinifera (and the first one I've found anywhere).

References

Small Western Green Drakes

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