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The Specimen

Ephemerella excrucians (Pale Morning Dun) Mayfly DunEphemerella excrucians (Pale Morning Dun) Mayfly Dun View 3 PicturesSize: 10mm. At emergence the specimen was a fairly bright olive green and there was obvious difference in color between the forewing (med. dun) and the hind-wing (pale cream). It was really noticeable as they floated by. You can just make these features out in the second photo, but not so much in the first that was taken 24 hours after capture. Total time from emergence to molting - approx. 48 hours.


Edit 2/25/13 - This specimen was originally posted to E. d. infrequens because of its size. It turns out large size doesn't hold up as a way to tell these two apart. This is because excrucians has much greater variability than previously understood. The assumption by anglers that excrucians is always the smaller of the two is apparently not supported by science. There is a lot left to sort out with western Ephemerella species. This may include new discoveries and/or synonyms (Synonym: A former name of a taxon, usually a species. Entomologists frequently discover that two insects originally described as different species are one in the same, and they drop one of the names. The dropped name is said to be a synonym of the remaining name. These changes take a while to trickle into the common knowledge of anglers; for example, Baetis vagans is now a synonym of Baetis tricaudatus.) as well as reportage on new intraspecific variations broadening the descriptions of recognized species. Based on this specimen's Fall maturity, the best guess is that it is an unusual form of excrucians.

As to color, both species duns (nymphs too) demonstrate a tremendous amount of intraspecific variability from pale yellow to bright green with a multitude of sulfur shadings in between, ranging from pale amber, through orange to cinnamon and even dark brown. I've seen wings from pale cream through tannish and almost every shade of dun except the dark shades. Some have pigment stained leading edges matching their bodies, some don't. Most of these variations are undocumented except in angler references and periodicals. It seems a rare year that a new variation doesn't pop up to the notice of anglers.

Bottom line - size is only reliable if the specimens are smaller than size 16, pointing to excrucians. Otherwise, the only fairly dependable way to tell them apart (especially the females) is by timing as infrequens is the first of the two to appear, rarely lasting longer than a couple of weeks or later than the end of June most years. The problem with using timing for determination is it requires knowledge of the hatch sequences as they actually occurred for a given year on a given piece of water. Obviously, this kind of information is seldom available. Without it, determining between the two duns if they are larger than size 18 is speculative at best - at least until very late in the Summer.
Collected October 16, 2011 from the Fall River in California
Added to by on October 21, 2011

The Discussion

MartinlfJanuary 12th, 2012, 8:10 pm
Palmyra PA

Posts: 3233
Thanks for adding these!
"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell
EntomanJanuary 13th, 2012, 1:30 am
Northern CA & ID

Posts: 2604
Hi Louis,

Your welcome, and thanks for the compliment. Due to Jason and his tutoring, some of them are worth looking at. Hopefully over the next few years, a lot more western specimens will be added to the encyclopedia to broaden its representation. That's my intent anyway. I've actually added quite a few already, but for some reason they didn't show up on the forum as new listings like they used to. That's probably a good thing since they would have dominated the board and thrown a lot of good topics off it prematurely. Perhaps I should go back and start a topic with one of them from time to time so guys can have a chance to see them? They're in the encyclopedia, but you'll only see them if by chance you happen to be looking up a particular species. Personally, I'd like to see new ones as they come in by the contributors. What do you think?


"It's not that I find fishing so important, it's just that I find all other endeavors of Man equally unimportant... And not nearly as much fun!" Robert Traver, Anatomy of a Fisherman
EntomanJanuary 13th, 2012, 4:40 pm
Northern CA & ID

Posts: 2604
Personally, I'd like to see new ones as they come in by the contributors. What do you think?

Sorry, didn't mean to imply I was only interested in Louis's thoughts. Any other opinions out there?
"It's not that I find fishing so important, it's just that I find all other endeavors of Man equally unimportant... And not nearly as much fun!" Robert Traver, Anatomy of a Fisherman

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