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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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Lateral view of a Male Ephemerella subvaria (Ephemerellidae) (Hendrickson) Mayfly Dun from the Beaverkill River in New York
I collected this male Hendrickson dun and a female in the pool on the Beaverkill where the popular Hendrickson pattern was first created. He is descended from mayfly royalty.
Jpsully
bedminster, nj

Posts: 4
Jpsully on Jan 10, 2007January 10th, 2007, 6:59 am EST
Jason:

Looking at the fifth photo from the top (the one with the ruler under the fly), it would appear that if you were to rotate the fly just a bit to the left, it would show that the fly is over one inch (25mm) long. That would make it twice as long as any Hendrickson I have ever seen. The average Hendrickson (to the best of my knowledge) is usually around 10-12mm (or about 1/2 inch), hook size 10-12. The hook size just doesn't seem to fit either. Where am I going wrong when looking at this fly (and hook)?

JP
Martinlf
Martinlf's profile picture
Moderator
Palmyra PA

Posts: 3047
Martinlf on Jan 10, 2007January 10th, 2007, 7:42 am EST
JP, Unless I'm mistaken, I believe that the ruler Jason is using is metric, that each little increment is one mm. I then see the bug measured at about 9-10 mm in the photo (not counting, of course the tails), and size 12 fits your accurate observations about the typical Hendrickson hook size. Now, when and where will we start seeing this bug come spring? I'm ready now.
"He spread them a yard and a half. 'And every one that got away is this big.'"

--Fred Chappell
Troutnut
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Administrator
Bellevue, WA

Posts: 2758
Troutnut on Jan 10, 2007January 10th, 2007, 7:54 am EST
Louis is right: it's a metric ruler, and the fly's body is about 11mm long.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist

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