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The Specimen

Ephemerella aurivillii Mayfly NymphEphemerella aurivillii  Mayfly Nymph View 11 PicturesThis is a puzzling one to identify and I'm not sure about the species. The maxillary palp (
The palp on the maxilla of an Ephemerella nymph (detached and photographed under a microscope) is highlighted in red here.
The palp on the maxilla of an Ephemerella nymph (detached and photographed under a microscope) is highlighted in red here.
Palp: A long, thin, often segmented appendage which can protrude from certain insect mouth parts such as the maxillae. Also known as the < />palpus.
)
is present and segmented, and the maxillary canines are not strongly serrate laterally. I think it's Ephemerella, not Serratella. The ventral (Ventral: Toward or on the bottom.) lamellae of the gills on abdominal segment 6 have a clear median notch with a depth at least half the length of the lamellae, which points toward a couple of uncommon species (most likely Ephemerella alleni), but the abdominal tubercles (
A few (not all) of the abdominal tubercles on this Ephemerella needhami nymph are circled.  They are especially large in this species.
A few (not all) of the abdominal tubercles on this Ephemerella needhami nymph are circled. They are especially large in this species.
Tubercle: Various peculiar little bumps or projections on an insect. Their character is important for the identification of many kinds of insects, such as the nymphs of Ephemerellidae mayflies.
)
and coloration don't fit that species. To add to the confusion, none of the above species are expected to emerge in the fall, as far as I know. I'm going to call this one Ephemerella aurivillii for now, but that's highly uncertain.
Collected September 10, 2020 from the Foss River in Washington
Added to Troutnut.com by on September 19, 2020

The Discussion

MillcreekSeptember 24th, 2020, 9:28 pm
Healdsburg, CA

Posts: 344
Jason,
I think this nymph may be Ephemerella tibialis. I am basing this on the following papers by Allen and Edmunds.

http://www.ephemeroptera-galactica.com/pubs/pub_a/puballenr1963p583.pdf

http://www.ephemeroptera-galactica.com/pubs/pub_a/puballenr1965p244.pdf

Ephemerella tibialis is in the paper on Serratella.
"If we knew what it was we were doing, it would not be called research, would it?"
-Albert Einstein
TroutnutSeptember 26th, 2020, 10:53 am
Administrator
Bellevue, WA

Posts: 2592
I was using this more recent key from Jacobus et al (not sure where to find a PDF online at the moment):

Jacobus, L. M., N. A. Wiersema and J. M. Webb. 2014. Identification of Far Northern and Western North American mayfly larvae (Insecta: Ephemeroptera), north of Mexico. Joint Aquatic Science meeting, Portland, OR. 176 pp. + suppl. Unpublished workshop manual.

Here's the relevant key couplet:

81e) Maxillary canines strongly serrate laterally; thoracic nota with small, brown excrescences (sometimes difficult to detect), especially between forewingpads = Ephemerella (Vittapallia) tibialis (very similar to E. nuda)

81e’) Maxillary canines not obviously serrate laterally; thoracic nota without small, brown excrescences = 81f


I think I did find a number of E. tibialis nymphs earlier this year in Idaho. Here's one of them. I remember being able to make out the highly serrate canines pretty clearly and possibly the excresences, and I couldn't find either one on this specimen. However, I don't have the greatest microscope and am hoping to get a better one soon, so I might take a closer look.

Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist

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