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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.

Mayfly Species Penelomax septentrionalis

This fascinating mayfly is among the most distinctive in the Ephemerellidae family, because it has unusually long legs in both the nymphal and adult stages. The nymphs have a strikingly rugged, spindly appearance, and the adults are a strikingly deeper, amber-yellow color than other Ephemerellidae duns.

This species was overlooked in the fly-fishing literature for many years. The first details appear in Fly-Fishing Pressured Water, which states that this species (formerly called Ephemerella septentrionalis) sometimes supports hatches of Ephemerella invaria. Ernest Schwiebert's Nymphs Volume I: The Mayflies: The Major Species seconds that claim, suggesting that it generates fishable hatches which may go unnoticed due to confusion with both Ephemerella invaria and Ephemerella dorothea. However, the long legs, thin dorsal stripe, and more vivid color make it easy to tell them apart upon close inspection.

Where & when

Time of year : May through midsummer

There are records of this species from Tennessee north into Ontario and Quebec. They begin in early May near the southern end of their range, and continue into midsummer in Canada. Expect them from mid-May through early June in the popular fly fishing waters of Pennsylvania and New York, from the limestone spring creeks to the freestone Catskill and Poconos waters. Their timing coincides with the more common and similar Ephemerella invaria.

Hatching behavior

Time of day : Late afternoon early in the season; twilight later on

Spinner behavior

Time of day: Twilight

Nymph biology

Substrate: Weedy stretches of stream

These nymphs may be most abundant in very small streams, but they are known from larger rivers as well, and I have collected them from the main stem of the Delaware. Needham et al (1935) mentions that these nymphs are commonly found among grass roots.

Specimens of the Mayfly Species Penelomax septentrionalis

2 Nymphs
1 Female Dun
Female Penelomax septentrionalis  Mayfly Dun
I'm confident this dun belongs to septentrionalis, because her legs are just too long for any other Ephemerellid, and her unusual mid-dorsal stripe matches those of two easily identified nymphs I collected some miles downstream.

This really pretty mayfly was in kind of bad shape when I found it crippled on the surface, and bouncing around in my container with a bunch of green drakes didn't help.

Recent Discussions of Penelomax septentrionalis

Anyone know more about Ephemerella septentrionalis?
11 replies
Posted by Troutnut on May 18, 2007
Last reply on Jul 18, 2011 by Oldredbarn
I found about one sentence on these in Gonzo's book, and haven't seen them mentioned anywhere else in fly fishing literature. Nor is any of the scientific literature I have on them particularly interesting (just descriptions). Now that I've collected a few and see what unique-looking nymphs they've got, I'm really curious about them.

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