Mayfly - Stenonema femoratum

Mayfly - Stenonema femoratum [3]
Bugguide.net links Ephemeroptera (Mayflies) » Heptageniidae (Stream Mayflies) » Stenonema » Stenonema femoratum
Members of this species are commonly called "Cream Cahills."
In fly fishing, adults are called spinners, the winged immatures are called duns.
Live adult mayflies photographed in the wild at Warrenville, Illinois, USA.

Mayfly - Stenonema femoratum
Members of this species are commonly called "Cream Cahills."

Ephemeroptera, Greek Ephemeros - short-lived, pteron - wing, referring to the short life span of adults. "As winged adults, they survive only a few hours or at most a few days. They eat nothing, nor do they crawl or walk. They only fly and mate within dancing swarms, usually in late afternoon or evening. Swarms, consisting of hundreds or thousands, emerge from the water after synchronously appearing along and inland of the shoreline.

Mayflies constitute one of the most important groups of bottom-dwelling animals in streams and rivers throughout the world. They are also found in ponds and shallow lake areas. While the larvae (or nymphs) of mayflies live in water, the adults (and subimagos) are delicate flying forms noted for their often ephemeral lives. Mayflies are routinely used for monitoring water quality because their presence and diversity can be valuable indicators of the health of their aquatic environment.

Mayfly - Stenonema femoratum

"This is a male Stenonema femoratum imago. The distinctive traits are tightly clustered cross veins near the bulla of the forewing in combination with three dark marks along the posterior edges of the terga." -- Lloyd Gonzales, 1 August, 2009 - Bugguide.net

From Troutnut.com:
"These are often called Cream Cahills.

This is the only species remaining in the Stenonema genus after the classic superhatches were reclassified into Maccaffertium. Its habitat and behavior is not different from the Maccaffertium species, except as noted below, and you should consult the page for that genus if you need angling information for this hatch." [4]

Mayfly - Stenonema femoratum
Mayflies in this genus are known for having nymphs which prosper in both cold and warm water. They are most often talked about as denizens of rivers and
streams; this specimen was photographed next to a pond, about 300 yards from the nearest moving water.

Lacrosse, Wisconsin
Mayfly Hatches Show Up on Weather Radar

The National Weather Service said it had recorded on its Doppler radar a gigantic cloud of recently hatched mayflies along the Mississippi River near LaCrosse. According to a spokesman, the flies started hatching about 9 p.m. on June 9th and continued overnight along the river valley.

According to the NOAA NWS, "Some roads across the Mississippi in Lacrosse were covered with them, piling into 'drifts' on bridges across the river and its tributaries. Local businesses with high intensity lighting found large piles of mayflies accumulating under the lights by midnight."

Many areas of the country, including Great Lakes Huron and Erie have experienced record-setting mayfly hatches in recent years, said Mike Miller, a stream ecologist with the Wisconsin Department of Natural Resources. The big hatches are probably indicative of improving water quality in the Mississippi and other waters, he said.

A similar event occured along the shores of Lake St. Claire and other waters in Michigan in 2001. Huge swarms of mayflies in the genus Hexagenia were caught on doppler radar. A dramatic series of radar echo returns showing the growth of the swarms can be seen HERE. [1]

NOAA NWS radar image showing evolving clouds of mayflies
(July 2006)

Mayfly - Stenonema femoratum

These are some strange and alien-seeming and beautiful creatures. I wish we could blow one up to the size of, say, a horse and walk around it. Well, maybe not.
Actually, that would be scary.

The earliest certain mayfly nymphs in the fossil record are from the late Carboniferous. The difficulty of placing fossil nymphs in an insect order, however, make this in dispute, with some authors claiming appearance as early as the Devonian is possible. Most fossil mayflies are larval fossils, ostensibly due to the very short time they have in the adult stage. [6]

The Carboniferous is a geologic period and system that extends from the end of the Devonian period, about 359.2 ± 2.5 Mya (million years ago), to the beginning of the Permian period, about 299.0 ± 0.8 Mya. [5]

Description: Soft bodies with short setaceous (bristle-like) antennae and vestigial mouthparts; wings held vertically at rest, hind pair much reduced; intercalary veins and many crossveins present; abdomen with long cerci, and with or without a medial caudal filament; larvae (nymphs) aquatic, campodeiform (elongated and flattened) with tracheal gills of varied form; true adult preceded by a subimago (winged instar).

Life Cycle: Mating normally occurs the same day adulthood is achieved. Females release as many as 8,000 fertile, oval eggs over the water, often scattering them or, in some species, in mass in a suitable place. After eggs are laid, females fall to the water and float, often drifting onto beaches in nuisance piles or windrows. Others are strongly attracted to and congregate under night lights.

Eggs laid on the water surface gradually sink to the bottom and, after a few days or several months, hatch into tiny aquatic nymphs well adapted for living at the bottom of quiet bodies of water or rapidly flowing streams. Some species burrow into the lake sediment to feed on algae, diatoms, aquatic vegetation, other aquatic insects, etc. When mature, nymphs swim to the surface or climb up plant stems or rocks where they break the nymphal skin, wait briefly for the wings to dry, and fly off. (This subimago period lasts a few minutes to 48 hours, depending on the species.) Subimagos are dull in appearance while true adults are shiny with longer tails and legs. [1]

Anglers go to great lengths to identify hatch periods, and there are hundreds of different types of both wet and dry artificial flies for dozens of different species, water conditions, adults, nymphs, you name it. The flies have fanciful names such as Blue Wing Olive Hackle Stacker Sparkle Dun, Green Drake Loop Wing Paradun and parachute creampuff.

From FlyFishUSA.com:
"Parachute Cream Puff
This fly is very popular for imitating Pale Morning Dun Mayflies on our local rivers on both sides of the Cascades. Hatches can start in early May and continue through August. This easy to see parachute fly was first ordered as a custom tie by Dick Crossley and was known as the Crossley Cream Puff."

Please also see an important article at Midcurrent.com Cream Cahills and Light Cahills (.pdf) for an extensive overview of the mayflies in general and this species in particular, through the eyes of heavy-duty fly fishermen.

References

National Oceanic and Atmospheric Administration, U.S. Department of Commerce
Troy Bartlett, Bugguide.net, Ephemeroptera
Bugguide.net, Stenonema femoratum
Knopp, Malcolm, and Robert Cormier. Mayflies: An Angler's Study of Trout Water Ephemeroptera. 1st ed. The Lyons Press, 2001.
Wikipedia, the free encyclopedia, Carboniferous
Fossilemuseum.net Ephemeroptera Fossil Insects

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