Mayfly Genus Siphlonurus (Gray Drakes)

Where & when

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.

In 215 records from GBIF, adults of this genus have mostly been collected during June (46%), July (27%), May (12%), and August (12%).

In 361 records from GBIF, this genus has been collected at elevations ranging from -30 to 11444 ft, with an average (median) of 804 ft.

Hatching behavior

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:


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

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

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.