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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

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 Neoleptophlebia (Leptophlebiidae) Mayfly Nymph from the Yakima River in Washington
Some characteristics from the microscope images for the tentative species id: The postero-lateral projections are found only on segment 9, not segment 8. Based on the key in Jacobus et al. (2014), it appears to key to Neoleptophlebia adoptiva or Neoleptophlebia heteronea, same as this specimen with pretty different abdominal markings. However, distinguishing between those calls for comparing the lengths of the second and third segment of the labial palp, and this one (like the other one) only seems to have two segments. So I'm stuck on them both. It's likely that the fact that they're immature nymphs stymies identification in some important way.
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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Species Ephemerella subvaria (Hendricksons)

Lateral view of a Male Ephemerella subvaria (Ephemerellidae) (Hendrickson) Mayfly Dun from the Beaverkill River in New York
The Hendrickson hatch is almost synonymous with fly fishing in America. It has been romanticized by our finest writers, enshrined on an untouchable pedestal next to Theodore Gordon, bamboo, and the Beaverkill.

The fame is well-deserved. Ephemerella subvaria is a prolific species which drives trout to gorge themselves. Its subtleties demand the best of us as anglers, and meeting the challenge pays off handsomely in bent graphite and screaming reels.

Where & when

Time of year : April-May

This hatch marks the start of serious mayfly action on almost every major trout water in the East and Midwest in the early spring.

The emergence begins in early April in the trout streams of southern Pennsylvania. It peaks in mid-May in the Catskills and late May farther north. It can linger through early June in northern areas like the Upper Peninsula of Michigan.

Good action lasts from two to three weeks on any given river.

In 41 records from GBIF, adults of this species have mostly been collected during May (44%), June (27%), and April (10%).

In 20 records from GBIF, this species has been collected at elevations ranging from 295 to 2372 ft, with an average (median) of 400 ft.

Species Range

Hatching behavior

Time of day : Afternoon, usually mid-afternoon, for about 1 hour

Habitat: Calm water with slow to medium current

Water temperature: Most sources say 50-55 °F, though Fred Arbona reports 55-65 °F

Hendricksons emerge by crawling out of their nymphal skins in the surface film, and emerger patterns are ideal. The duns float for a long time before taking flight and they, too, can be important to imitate. The Hendrickson and Red Quill fly patterns were created to meet this demand in the early days of American fly fishing.

Spinner behavior

Time of day: Late afternoon to evening; or, on cold evenings, delayed until 7-11:00 a.m. the next day

Habitat: Riffles and runs

Most sources say that the females drop their eggs from high above the water and become vulnerable to trout only when they fall spent with the males. These can be intense events, especially on warm days when the activity is compressed into a shorter time period, about 30 minutes, near dusk.

Knopp & Cormier in Mayflies: An Angler's Study of Trout Water Ephemeroptera contradict the more common story. They acknowledge that sometimes the females drop their eggs from on high, but they claim that Hendricksons more often fall to the water, not yet spent, before releasing their eggs. They report that trout may be selective to spinners with upright, half-spent, or spent wings. Some fly shops sell patterns with bright butts to imitate these egg-laden females.

Nymph biology

Current speed: Everything except extremely fast; they move to slower water near emergence time.

Substrate: Gravel, small boulders, vegetation. Swift gravelly runs are especially favored.

Environmental tolerance: Among the least tolerant species in the family. They require moderate temperatures, neither too hot nor spring-fresh cold, and they cannot tolerate pollution.

Dorsal view of a Ephemerella subvaria (Ephemerellidae) (Hendrickson) Mayfly Nymph from Salmon Creek in New York
There is tremendous physical variation within this species, especially in the coloration of the nymphs. In Hatches II, Caucci and Nastasi describe six distinct variations on Hendrickson nymphs, and my collections show at least one more. The lesson is that the angler should study the nymphs from his own stream and carry varied imitations.

The nymphs are very active and available to trout in the hours leading up to and during the hatch, and sometimes they negate the importance of the duns. This is especially likely on cold days, according to Knopp & Cormier, who recommend fishing a wiggle nymph under such circumstances. Several authors have observed that the nymphs swim with a strong wiggle when in slow water, but they sit back and dead drift in faster currents. Our fishing should reflect this.

Before the hatch, the nymphs repeatedly swim up and down from the surface in what seem like "practice runs" for emergence. Sometimes they do this days or even weeks before emerging; I have seen this behavior in northern Wisconsin on March 17th, over a month before the first emergence on the river.

Ephemerella subvaria Fly Fishing Tips

Comparaduns, emergers, floating nymphs, and wiggle nymphs are especially recommended for this hatch. The angler should also carry a variety of patterns to match the important spinners.

There are significant differences between the males and females in both size and color, and anglers should be prepared to match either one.

Wet or cloudy days are traditionally regarded as good Hendrickson weather, although I have fished them under a wide array of conditions, from snowstorms to warmth and sun. If you encounter them on especially damp or stormy days, you've got good reason to try fishing a cripple pattern. A crippled Hendrickson dun imitation might work just as well on a calm, bright, spring day as on a stormy one, but that gives us one less excuse to feel clever about choosing to fish one.

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Male Spinner

Body length: 9 mm
Wing length: 10 mm

A species of the Ephemerella invaria group; close to E. invaria, but distinguished by the prominent dark veins and cross veins; nymph rather similar to E. rotunda (now a synonym of E. invaria), but with distinct dorsal spines, and often paler in color.

Eyes of living male bright red brown. Head deep brown. Thorax deep brown, with reddish tinges on the pleura. Legs deep dull amber; fore femur tinged with smoky, middle and hind femora often with traces of reddish median shading. Wings hyaline; longitudinal and cross veins distinctly light brown; no cross veins in the basal two thirds of the costal and radial spaces.

Abdomen light reddish brown dorsally; shaded laterally with smoky; posterior margins smoky. Ventrally paler reddish brown, the apical sternites somewhat darker. Lateral margin of segments yellowish. Forceps dull yellowish. Penes very similar to Ephemerella invaria; apical spines usually absent. Tails pale, yellowish at base; joinings distinctly smoky brown.


Nymph lightly smaller than E. rotunda (now a synonym of Ephemerella invaria), and paler in color; tergites 6 and 7 often almost entirely pale, while 5 has only a dark median subtriangular patch; tergites 8 and 9 always dark. Dorsal spines present on abdomen, much better developed than in rotunda; lateral extensions much as in rotunda. Pale dorsal spots at base of spines not present.

Specimens of the Mayfly Species Ephemerella subvaria

7 Male Duns
5 Female Duns
2 Male Spinners
4 Female Spinners
16 Nymphs

4 Streamside Pictures of Ephemerella subvaria Mayflies:

18 Underwater Pictures of Ephemerella subvaria Mayflies:

Discussions of Ephemerella subvaria

Hatching Hendrickson
Posted by Martinlf on Oct 14, 2008
Last reply on Oct 14, 2008 by Martinlf
Here's another hatching mayfly, this one stillborn. Click on "31 more specimens" and scroll down.
When does a Hatch happen?
8 replies
Posted by TheMidge on May 10, 2007
Last reply on May 26, 2007 by Greenghost
I was hoping I could get a little info on what effects a Hatch. Air Temp? Water Temp? A combination? I'm trying to determine the best time to catch a hatch on stream near me, as I have a small window to fish in on a short trip home. I am expecting to see Hendrickson's, as it's the right time of year (or so I have been told) for this stream. What's the best way to guess?
underwater photos
4 replies
Posted by Martinlf on Dec 26, 2006
Last reply on Apr 21, 2007 by PeterO
Jason, thanks for the underwater photos of subvaria nymphs and the stillborn dun. Anyone looking at this thread may have to search for them a bit, (click on "There are 29 more specimens") but they are well worth the viewing! They have given me a better understanding of how to modify my upside down mayfly tie to better represent still born and crippled subvarias, and the underwater nymph pictures have confirmed my thoughts about coloration on flies designed to imitate subvaria nymphs. The photos are phenomenal, not like any bug photos I've seen before, in that they show the insects in a natural habitat.

Start a Discussion of Ephemerella subvaria


Mayfly Species Ephemerella subvaria (Hendricksons)

Species Range
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