Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large Brown Trout that only touch streamers for the rest of the year.

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Mayfly Species Siphlonurus occidentalis (Gray Drake)

This is the main Western species of Siphlonurus, and probably the most productive species of that genus in America.

Where & when

Time of year : April to October; peaking in August and September

Altitude: 3,000 to 5,000 feet

This species most abundant in the late summer and early fall when few other mayflies are on the water. Its activity may last few months in a single location. There's a good chart in Knopp and Cormier's Mayflies: An Angler's Study of Trout Water Ephemeroptera detailing where and when to find this species.

Hatching behavior

Time of day : Midday

The emergence is relatively unimportant because the duns usually cawl out onto shore to hatch, but shallow nymphs may be effective when this activity is at its peak.

Spinner behavior

Time of day: Mid-morning or evening

The spinners are the most important stage of this hatch.

Specimens of the Mayfly Species Siphlonurus occidentalis

2 Male Spinners

Male Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

6 Pictures

Collected July 31, 2020 from the Henry's Fork of the Snake River in Idaho

Added by Troutnut on August 17, 2020

Male Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

6 Pictures

This specimen was easy to key to species using the old keys in < id="115" />, and was the basis for IDs of several other Siphlonurus specimens of the same species collected on the same day.

Collected July 31, 2020 from the Henry's Fork of the Snake River in Idaho

Added by Troutnut on August 16, 2020

2 Female Spinners

Female Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

9 Pictures

Collected July 31, 2020 from the Henry's Fork of the Snake River in Idaho

Added by Troutnut on August 16, 2020

Female Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

5 Pictures

This specimen was identified by association with this male.

Collected July 31, 2020 from the Henry's Fork of the Snake River in Idaho

Added by Troutnut on August 16, 2020

1 Spinner

Siphlonurus occidentalis (Gray Drake) Mayfly Spinner

4 Pictures

Collected July 31, 2007 from Crazy Beaver Spring in Montana

Added by Bnewell on June 28, 2011

1 Female Dun

Female Siphlonurus occidentalis (Gray Drake) Mayfly Dun

6 Pictures

I caught this specimen two days' walk from my macro lenses, but I don't think I have a dun of this species yet, so I took the best photos I could with my iPhone.

Collected August 30, 2020 from Island Lake in Wyoming

Added by Troutnut on September 18, 2020

2 Nymphs

Siphlonurus occidentalis (Gray Drake) Mayfly Nymph

2 Pictures

Collected August 4, 2011 from Rainbow Lake in Washington

Added by Bnewell on August 4, 2011

Siphlonurus occidentalis (Gray Drake) Mayfly Nymph

2 Pictures

This species emerges very late in the fall often along with Siphlonurus autumnalis just before ice forms on edges of streams.

Collected November 30, 2006 from Crazy Beaver Spring in Montana

Added by Bnewell on June 28, 2011

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References

Arbona, Fred Jr. 1989. Mayflies, the Angler, and the Trout. Nick Lyons Books.
Caucci, Al and Nastasi, Bob. 2004. Hatches II. The Lyons Press.
Knopp, Malcolm and Robert Cormier. 1997. Mayflies: An Angler's Study of Trout Water Ephemeroptera . The Lyons Press.
Schwiebert, Ernest G. 1955. Matching the Hatch. MacMillan Publishing Company.
Swisher, Doug and Carl Richards. 2000. Selective Trout. The Lyons Press.