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Ameletus vernalis (Brown Dun) Mayfly Nymph Pictures

 I keyed this one out using the Alberta species key from Zloty & Pritchard 1997.

Notes from the ID include:

1. Posterior (Posterior: Toward the back of an organism's body. The phrase "posterior to" means "in back of.") margin of sternites (
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
One sternite of this Isonychia bicolor mayfly spinner is highlighted in red.
Sternite: The bottom (ventral) part of a single segment on an insect's abdomen.
)
6-8 without spines
2. Mesal (Mesal: Toward the middle.) gill extension "well developed"
3. Basil third of caudal (Caudal: Toward the posterior tip of the body.) filaments pale
4. Anterior (Anterior: Toward the front of an organism's body. The phrase "anterior to" means "in front of.") surface of front femora (
The femur of this Isonychia bicolor mayfly spinner is highlighted in red.
The femur of this Isonychia bicolor mayfly spinner is highlighted in red.
Femur: The main segment of an insect's leg close to the body, in between the tibia and the trochanter.
)
mostly pale
5. Dark band on caudal (Caudal: Toward the posterior tip of the body.) filaments begins around segment 20
6. Final instars (Instar: Many invertebrates molt through dozens of progressively larger and better-developed stages as they grow. Each of these stages is known as an instar. Hard-bodied nymphs typically molt through more instars than soft-bodied larvae.) early season

This one keys out pretty cleanly to Ameletus vernalis except the color pattern on the tergites (
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.
)
doesn't match. However, two species known in Washington aren't included in the key. Of those two, Ameletus vancouverensis would be too small (adult body length 9 mm), but Ameletus andersoni (currently documented only from a spring in Cowlitz County) has a wide range of sizes and emergence times that could be compatible with this one. So I can't rule that species out, but Ameletus vernalis seems the most likely.


This mayfly was collected from the Yakima River on April 9th, 2021 and added to Troutnut.com on April 12th, 2021.


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