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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.

Mayfly Genus Siphlonurus (Gray Drakes)

The important Eastern species are Siphlonurus quebecensis, Siphlonurus alternatus, and to a lesser extent Siphlonurus rapidus. All may produce fishable spinner falls, often with more than one species in the same swarm, but these are generally localized and minor events compared to the classic superhatches.

The main Western species, Siphlonurus occidentalis, is much more valuable. Its importance in the West is often compared to that of Isonychia in the East.

Where & when

Preferred waters: Slow, alkaline streams and lakes

Siphlonurus hatches are generally slow-paced events, and it takes two to three months for the entire generation to hatch out of a given stretch of water. See the species pages for emergence details about each one.

Hatching behavior

Time of day : Morning through midday

Water temperature: Varies, but 60-63°F is ideal

There is some disagreement about exactly how Siphlonurus duns hatch. It is certain that they crawl out onto shore sometimes and hatch on the surface at other times, but which of these behaviors is more prominent is anybody's guess.

The time of emergence is also up in the air; most say it is in the morning, but there are reports of emergence in the afternoon and in the middle of the night, as well. One account suggests it is most common in the evening.

Authors generally agree that, regardless of hatching mode or time, the duns are the least important stage of this species. There are reports of trout feeding well on them, and this is sometimes attributed to the wind blowing the duns into the water.

In Mayflies Ted Fauceglia relays a detailed account by Dick Stewart of the hatching of Siphlonurus duns, quoted here in part:

They then emerge by swimming toward the surface and immediately taking flight. They pop through the surface film like miniature helicopters rising toward the open sky. We believe that molting takes place either very quickly or when the duns are high in the trees, for try as we might, we almost never can find a dun (day or night) despite the fact that we can locate hundreds of spinners.

Another point on which most agree is that Siphlonurus duns emerge from very shallow margins and backwaters, sometimes far from the trout in the main channel.

Spinner behavior

Time of day: Usually dusk in the East and Midwest, morning in the West, sometimes afternoon in cold weather

Gray drakes return as spinners two to four days after hatching as duns, and they gather in swarms over the riffles. Accounts of their ovipositing behavior vary: some say the females repeatedly dive and dip their abdomens briefly into the water, while others say they drop their eggs from the air. Both genders eventually fall spent, but the spinner falls may be tricky to locate because the females sometimes fly far upstream after mating but before ovipositing. Trout may feed on Siphlonurus spinners very selectively.

Nymph biology

Current speed: Slow

Substrate: Silt, vegetation

Siphlonurus nymphs are not completely intolerant of medium to fast current, but their best populations are found in slow streams or still waters, often with plenty of weed growth. They are more frequently important to fly fishers than the duns.

They are extremely adept swimmers.

Specimens of the Mayfly Genus Siphlonurus

4 Male Spinners
6 Female Spinners
1 Spinner
10 Nymphs
2 Female Duns
1 Adult

3 Underwater Pictures of Siphlonurus Mayflies:

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References

Taxonomy
Species in Siphlonurus
Siphlonurus alternatusGray Drake29
Siphlonurus autumnalisGray Drake310
Siphlonurus columbianusGray Drake00
Siphlonurus occidentalisGray Drake840
Siphlonurus phyllisGray Drake24
Siphlonurus quebecensisGray Drake736
Siphlonurus rapidusGray Drake00
Siphlonurus spectabilisGray Drake00
12 species (Siphlonurus barbaroides, Siphlonurus barbarus, Siphlonurus decorus, Siphlonurus demaryi, Siphlonurus luridipennis, Siphlonurus marginatus, Siphlonurus marshalli, Siphlonurus minnoi, Siphlonurus mirus, Siphlonurus noveboracana, Siphlonurus securifer, and Siphlonurus typicus) aren't included.
Genus Range
Common Names
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