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Insect Order Ephemeroptera (Mayflies)

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Mayflies may be the most important insects for trout anglers to understand. They are an ancient order of insects, famous outside the fly-fishing world for their fragile beauty and short adult lifespan, often a single day to mate and die. The mayfly's poignant drama attracts poets and fishermen alike, but fishermen make the most of it.

Mayflies live more than 99% of their lives as nymphs on the river or lake bottom, filling many crucial roles in freshwater ecosystems as they feed and grow. They eventually emerge from the water as winged sub-adults called "subimagos" by scientists and "duns" by anglers. Duns evolved to be good at escaping the water, with a hydrophobic surface and hardy build, but they are clumsy fliers. Within a day or two they molt one last time into "imagos" or "spinners," the mature adults, a transformation captured in this photo series of a dun molting into a spinner. They have longer legs and tails, and sleeker, more lightweight bodies, giving them the airborne speed, agility, and long grasp they need for their midair mating rituals. They are usually darker than the duns and have shinier, more transparent wings. They die within minutes or hours after mating.

Hatching Behavior


The importance of mayflies comes largely from their emergence and mating behavior. While many organisms assure the survival of their species using individualistic tools like stealth, speed, venom, or parental care, mayflies are famous instead for their "strength in numbers" approach. They coordinate their emergence and mating times (both time of year and time of day) so that they leave their safe habitats and emerge together in large numbers in a very short period of time. This can trigger feeding frenzies in every nearby insect-eater, from trout to birds to dragonflies, but there are simply so many mayflies at once that many luck out and survive to reproduce. These trout feeding frenzies are the stuff of legend among fly anglers, and they also pose one of our greatest challenges, because trout feeding feverishly on a thick hatch are often unwilling to strike any fly that doesn't properly imitate the mayfly of the hour.

The duns of each species typically emerge during an hour or two each day for a couple of weeks in the spring or summer, though there are some important fall hatching species. Some species follow more sporadic emergence strategies, and many of these combine to create a sort of "background noise" of miscellaneous mayfly activity on many trout streams throughout much of the summer. This mixed bag of mayflies provides good opportunities for anglers to catch rising trout that aren't too picky.

Mayfly nymphs emerge into duns in several different ways. Most often, the nymph swims to the water's surface and splits open its exoskeleton above the thorax (Thorax: The thorax is the middle part of an insect's body, in between the abdomen and the head, and to which the legs and wings are attached.). The dun wriggles out onto the surface, where its wings fill with fluid hydraulically and allow it to take flight. (In contrast to the common myth of mayflies needing time to "dry their wings," this process is more like inflating a raft.) Different species may be quick or slow at each stage of this process. Some take a long time to escape their nymphal shuck (
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Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.), making flies that imitate these "emergers" especially effective. Some species very quickly take flight when they hit the surface, while others ride the surface for some minutes like little sailboats, a prime target for hungry trout and a welcome sight for the dry fly angler. Cool or windy weather may prolong these struggles and increase the availability of mayflies to trout.

Many important species follow completely different emergence patterns. In some, full winged duns emerge on the bottom of the stream and float to the surface. Others swim to shore and crawl out on land before emerging. Learning to identify mayflies and associate them with the right behaviors gives an angler an advantage: the ability to make a good guess about which style and stage of emergence to imitate, simply from seeing and recognizing some duns or mature nymphs.

Spinner Behavior


Once mayflies have molted into spinners (imagos), they usually gather in swarms to mate, usually over riffles. When they're done they fall dead, or spent (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.), on the water in an event anglers call a spinner fall. Spinner falls are usually better coordinated than emergences, because spinners gather in swarms for mating. This means some species with sporadic, unnoticed dun emergences become far more concentrated and important to anglers as spinners.

Spinner falls are also usually more predictable than emergences, because so many of them (although not all) take place at dusk, and they are preceded by visible aerial spinner swarms, which may start hours earlier at treetop level and descend gradually toward the water as night falls. Dusk spinner falls often mark the angler's best chance to see a good rise of trout each day. However, like most things in nature, mayfly spinners aren't as predictable as we'd like. Sometimes clouds of thousands of spinners will gather over a riffle in the evening and fly back into the woods as quickly as they came, never falling spent (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.). When that happens, anglers must swallow their disappointment and look for them to finish the job in the morning.

Mayfly females face the extra duty of laying their eggs after mating. Many species release their eggs as they fall spent (Spent: The wing position of many aquatic insects when they fall on the water after mating. The wings of both sides lay flat on the water. The word may be used to describe insects with their wings in that position, as well as the position itself.) on the water near the males after mating. Some land on the water, release a few eggs and take off again. Others fly low over the water and dip the tips of their abdomens below the surface for just a moment to release eggs. Still others drop their eggs from high in the air. In one very common genus, Baetis, the females land near shore and crawl underwater to lay their eggs in neat little rows on rocks and logs.

Nymph Biology


Mayfly nymphs or "naiads (Naiad: Naiad is the technical term for nymph used by modern entomologists.)" grow underwater for a period ranging from 3 months to 2 years, depending on the species. Like the stoneflies of Plecoptera, their development follows "incomplete" metamorphosis, meaning they do not undergo a dramatic transformation to adulthood via a pupal stage like butterflies and caddisflies (Trichoptera) do. Their changes are more gradual, at least internally. As the nymphs grow they proceed through numerous slightly different developmental stages called instars (Instar: Many invertebrates molt through dozens of progressively larger and better-developed stages as they grow. Each of these stages is known as an instar. Hard-bodied nymphs typically molt through more instars than soft-bodied larvae.), between which they molt out of their old exoskeletons and expose new ones.

Anglers recognize four categories of mayfly nymphs: swimming, burrowing, clinging, and crawling:

  • Some streamlined swimmers move like little bullets, faster then fish of the same size, and they swim upstream against strong current without a problem. Others inhabit slow water and use their speed to dart between leaves in the weed beds.

  • Clingers of the family Heptageniidae are typically flat nymphs with strong legs and claws for holding on to rocks. Some have evolved further adaptations for clinging in fast water; for example, the genus Rhithrogena has gills resembling suction cups. There is great variation among the clingers and some species have adapted to slow water.

  • Crawlers come in the most varied forms; they are a catch-all group for "average" families that usually excel at neither swimming nor clinging. The Hendricksons and Sulphurs of the Ephemerella genus are typical crawlers. There are tiny crawlers like Tricorythodes, and there are oddballs like Baetisca. The crawlers in Leptophlebiidae are quite good at swimming, and those in Drunella (especially Drunella doddsii) are quite good at clinging.

  • The distinctive burrowers of Ephemeridae (and the less important Polymitarcyidae) are pale nocturnal creatures which use tusks to carve U-shaped burrows into the river bottom, where they live most of the time. Their long yellow bodies and feathery gray gills make them unmistakable. Their hatches are some of the angler's favorites, especially the giant Hexagenia limbata flies of the Midwest and West or the Eastern Green Drakes, Ephemera guttulata.



Entomologists have a similar system, but even their line between categories is a blurry one. Some burrowers swim well, some crawlers cling well, and some families like Leptophlebiidae and Potamanthidae straddle the boundary between categories.

If you fish a fertile stream, watch the bottom ahead of you as you walk. Sometimes, especially in April and May, you'll see lots of mayfly nymphs in front of you swimming out of your way or scurrying to the undersides of rocks. You don't need to be down on all fours with a magnifying glass to see mayfly life underwater.

Pictures of 678 Mayfly Specimens:

Specimen Page:1234...69
Baetis (Blue-Winged Olives) Mayfly NymphBaetis (Blue-Winged Olives) Mayfly Nymph View 10 Pictures
Collected May 6, 2007 from Mongaup Creek in New York
Added to Troutnut.com by Troutnut on May 18, 2007
Male Ephemerella subvaria (Hendrickson) Mayfly DunMale Ephemerella subvaria (Hendrickson) Mayfly Dun View 9 PicturesI collected this male Hendrickson dun and a female in the pool on the Beaverkill where the popular Hendrickson pattern was first created. He is descended from mayfly royalty.
Collected April 19, 2006 from the Beaverkill River in New York
Added to Troutnut.com by Troutnut on April 22, 2006
Hexagenia limbata (Hex) Mayfly NymphHexagenia limbata (Hex) Mayfly Nymph View 9 Pictures
Collected June 8, 2005 from the Namekagon River in Wisconsin
Added to Troutnut.com by Troutnut on May 26, 2006
Specimen Page:1234...69

41 Streamside Pictures of Mayflies:

Streamside Photo Page:12345
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This Ephemerella invaria sulphur dun got stuck in its shuck trying to emerge. This isn't exactly a "natural" pose for a photograph, but it kind of shows what an emerger pattern could look like. In this picture: Mayfly Species Ephemerella invaria (Sulphur Dun). From the Neversink River in New York.
This Ephemerella invaria sulphur dun got stuck in its shuck (
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Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.) trying to emerge. This isn't exactly a "natural" pose for a photograph, but it kind of shows what an emerger pattern could look like.

In this picture: Mayfly Species Ephemerella invaria (Sulphur Dun).
StateNew York
LocationNeversink River
Date TakenMay 20, 2007
Date AddedJun 5, 2007
AuthorTroutnut
CameraPENTAX Optio WPi
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Often mayflies can be found on houses near the river. This one molted from a dun into a spinner on the outside of our kitchen window. Any lit dwelling near the river can attract a lot of mayflies at night. A good way to determine what's hatching is to visit a gas station (or anything else with bright lights) close to the river early in the morning. In this picture: Mayfly Family Baetidae (Blue-Winged Olives). From the West Fork of the Chippewa River in Wisconsin.
Often mayflies can be found on houses near the river. This one molted from a dun into a spinner on the outside of our kitchen window.

Any lit dwelling near the river can attract a lot of mayflies at night. A good way to determine what's hatching is to visit a gas station (or anything else with bright lights) close to the river early in the morning.

In this picture: Mayfly Family Baetidae (Blue-Winged Olives).
StateWisconsin
LocationWest Fork of the Chippewa River
Date TakenMay 26, 2005
Date AddedFeb 8, 2006
AuthorTroutnut
CameraCanon EOS 20D
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Here's an above-the-water view of a stillborn Ephemerella subvaria dun which I also photographed from below the water. In this picture: Mayfly Species Ephemerella subvaria (Hendrickson). From Mongaup Creek in New York.
Here's an above-the-water view of a stillborn (
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This stillborn Ephemerella subvaria dun is trapped in its shuck.
This stillborn Ephemerella subvaria dun is trapped in its shuck.
Stillborn: In fly fishing, a stillborn insect is one which got stuck in its nymphal or pupal shuck during emergence and floats helplessly on the surface instead of flying away. It is a specific class of cripple, although it is sometimes used interchangeably with that term.) Ephemerella subvaria dun which I also photographed from below the water.

In this picture: Mayfly Species Ephemerella subvaria (Hendrickson).
StateNew York
LocationMongaup Creek
Date TakenApr 19, 2006
Date AddedApr 22, 2006
AuthorTroutnut
CameraPENTAX Optio WPi
Streamside Photo Page:12345

67 Underwater Pictures of Mayflies:

Underwater Photo Page:1234...8
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Several Baetidae nymphs line up on a rock. In this picture: Mayfly Family Baetidae (Blue-Winged Olives). From Mongaup Creek in New York.
Several Baetidae nymphs line up on a rock.

In this picture: Mayfly Family Baetidae (Blue-Winged Olives).
StateNew York
LocationMongaup Creek
Date TakenApr 19, 2006
Date AddedApr 22, 2006
AuthorTroutnut
CameraPENTAX Optio WPi
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This is my favorite underwater picture so far. It shows a bunch of Simuliidae (black fly) larvae clinging to a rock and swinging in the fast current. There are also at least four visible mayfly nymphs, probably in the family Baetidae. In this picture: Mayfly Family Baetidae (Blue-Winged Olives) and True Fly Family Simuliidae (Black Flies). From Eighteenmile Creek in Wisconsin.
This is my favorite underwater picture so far. It shows a bunch of Simuliidae (black fly) larvae clinging to a rock and swinging in the fast current. There are also at least four visible mayfly nymphs, probably in the family Baetidae.

In this picture: Mayfly Family Baetidae (Blue-Winged Olives) and True Fly Family Simuliidae (Black Flies).
StateWisconsin
LocationEighteenmile Creek
Date TakenMar 19, 2004
Date AddedJan 25, 2006
AuthorTroutnut
CameraOlympus C740UZ
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This picture from below shows a stillborn Ephemerella subvaria (Hendrickson) dun drifting on the surface amidst a number of shed pupal skins from Brachycentrus caddisflies which were heavily hatching that day. In this picture: Mayfly Species Ephemerella subvaria (Hendrickson) and Caddisfly Species Brachycentrus appalachia (Apple Caddis). From the East Branch of the Delaware River in New York.
This picture from below shows a stillborn (
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This stillborn Ephemerella subvaria dun is trapped in its shuck.
This stillborn Ephemerella subvaria dun is trapped in its shuck.
Stillborn: In fly fishing, a stillborn insect is one which got stuck in its nymphal or pupal shuck during emergence and floats helplessly on the surface instead of flying away. It is a specific class of cripple, although it is sometimes used interchangeably with that term.) Ephemerella subvaria (Hendrickson) dun drifting on the surface amidst a number of shed pupal skins from Brachycentrus caddisflies which were heavily hatching that day.

In this picture: Mayfly Species Ephemerella subvaria (Hendrickson) and Caddisfly Species Brachycentrus appalachia (Apple Caddis).
StateNew York
LocationEast Branch of the Delaware River
Date TakenApr 19, 2006
Date AddedApr 22, 2006
AuthorTroutnut
CameraPENTAX Optio WPi
Underwater Photo Page:1234...8

Recent Discussions of Ephemeroptera

The Tully Trico season 9 Replies »
Posted by BioGriz on Jun 24, 2013 in the genus Tricorythodes
Last reply on Oct 1, 2013 by BioGriz
Preface: Any hatch where you are knee deep in trout rising and sipping bugs is awesome, regardeless if you are struggling to get a tight line. But on my home water the Tulpehocken Creek (Tully), which many of you from PA have fished before, Trico hatch brings problems...
1) Stressfull/unhealthy conditions for the trout. i.e. low water & high temps
2) A lot more fishing pressure from the "fair weather" community

I am a relative greenhorn compared to some of you who have decades of experience under your belt, and am just fishing for some general opinions here.

Had a great day at rebers using sze 22 trike spinner but i am noticing the tully temp is climbing. What kind of impact can we make on the stream? Is fishing in the morning and laying off the trout in the afternoon ok? or should the tully be left alone completely??

Thanks for your input!
ReplyBig Green River, Wisconsin, late September 1 Reply »
Posted by Admiralb on Sep 28, 2013 in the species Baetis brunneicolor
Last reply on Sep 30, 2013 by Entoman
I hosted two visiting delegates to the T.U. National Convention in Madison, WI. on September 25, 2013, taking them to the Big Green River in Grant County near Fennimore. There was a high overcast in the morning, and these mayflies [which I merely called "BWO's" and imitated with #16 parachute dries - thin olive bodies, gray wing posts and dun parachute hackle] - were active. Both of my guests raised browns fishing a gray nymph behind a #16 parachute dry BWO. As long as my flies catch fish, I don't need to be a detailed entomologist. Nevertheless, I do sincerely appreciate the detailed scientific info, because it helps me focus my flytying and fly selection. Thank you.
ReplyTrico Tips 47 Replies »
Posted by Martinlf on Jul 21, 2007 in the genus Tricorythodes
Last reply on Aug 12, 2013 by Martinlf
I'll start with a fly patterns, follow with a bit of what I think I know about Tricos (Entomologists, please offer corrections if needed), and close with a few questions.

I love designing different patterns for Tricos, partly to keep myself entertained, and partly to show the fish something new from time to time. Jason's photos and the opinions of some fussy fish have led me to tie an extra large thorax recently on all my Tricos. My old standby is a parachute tied reverse, with a high vis post over the bend of the hook, and grizzly hackle, with no tails. It's modeled on Al's Trico, which could be found on the Little Lehigh Flyshop website until Rod closed the shop. An internet search may provide images now. It's very visible and fish generally approve. My newest fly is a take off from one of Gonzo's (Lloyd Gonzales) patterns in his book Fly-fishing Pressured Water, and it also shows the influence of Al's Trico. Gonzo ties an upside down Trico on a wide gap hook using synthetic material for the wing. I tie this fly also, and it certainly does catch fish, but I recently tied a version with grizzly hackle, making an oversize thorax and palmering hackle over the thorax to create a full wing. I then clipped hackle from the top of the fly (which becomes the bottom, as this is an upside down fly) so that the fly would sit flat, upside down, on my tying table. A drop of Locktite brush-on super glue on the bare recently clipped thorax after darkening the hackle stem with black marker and the fly was done. (By the way, I put tails on this one to balance it [P.S. Later correction: this pattern doesn't need the tails. I've caught plenty fish now on a tailless version] .) It caught several fish the first time I tried it on a heavily fished stream.

I believe for some, if not most species of Tricos, males hatch at night, females in the morning, and that the spinners fall when the air temperature hits the upper 60's. This generally means that as the season goes on, spinners hit the water later and later. Sometimes by 7:00 am (or earlier) in the early summer, by 10:00 (or later) in the fall.

It's been unseasonably cool in the Northeast the past couple of days, and I would have gone out this morning but for taking my daughter to a midnight showing of The Order of the Phoenix (I just couldn't get up) but I'm wondering if the spinner fall happens later than normal on cool mid-summer mornings like today's. I hope to find out Monday, but am curious if anyone has experiences to share. Also, does anyone have an effective Trico pattern to share? I'm always looking for ideas.
ReplyGD Shuck 10 Replies »
Posted by Martinlf on May 29, 2013 in the species Ephemera guttulata
Last reply on Jun 2, 2013 by Crepuscular
Jason's photo of a GD shuck suggests that at hatch time the backs of the nymphs may be a greyish or grey olive color. Possibly useful information, if this is an accurate surmise.
ReplySomething like recurvata, but different. 8 Replies »
Posted by DayTripper on Apr 6, 2013 in the species Litobrancha recurvata
Last reply on Apr 14, 2013 by Oldredbarn
Okay, I'm at a local TU meeting and a friend tells me about a double secret mystery hex hatch. According to him, these hex are colored differently than our limbata, hatch two or three weeks earlier for about 3 days only (last week of May or the first week of June), and exhibit some other "different" behavior traits. Needless to say, I was interested in learning more. Fast forward to this morning when I picked one of the nymphs up from his house after he dug a sample up for me to take home. Its still alive, so I'm trying to figure out what body parts I need photographs of to share here to see what you guys think this guy is.

A quick look at the head and tusks is pointing me to Litobrancha recurvata, and the emergence period shown in Hatches II for them is about right (May 14 to July 3). The photo they have in their color plates looks very close to what I just finished photographing, but what I have looks different.

I took a couple dozen quick photos of it and uploaded them to my flickr account, wasn't sure if it was ok or necessary to upload them all here. Link to the gallery below. Please let me know if you'd like me to upload any of these here, or if you'd like a better shot of a certain feature, so you can see the full res photo for better detail. In my non-expert opinion, I would say this nymph is near full maturity, as its ginormous. I can't find my metric ruler, but it is ~50mm long. Thanks for any help.

http://www.flickr.com/photos/cerveniak/sets/72157633180659254/


Alex
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Top 10 Fly Hatches
BaetidaeEphemerellidaeHexagenia limbataBaetisChironomidaeHeptageniidaeIsonychiaHydropsychidaeEphemerellaEphemerella subvaria
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Eat mayflies.
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