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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 anglers alike, but anglers 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 689 Mayfly Specimens:

Specimen Page:1234...70
Male Hexagenia atrocaudata (Late Hex) Mayfly SpinnerMale Hexagenia atrocaudata (Late Hex) Mayfly Spinner View 12 Pictures
Collected July 24, 2005 from the Namekagon River in Wisconsin
Added to Troutnut.com by Troutnut on April 15, 2006
Male Baetis (Blue-Winged Olives) Mayfly DunMale Baetis (Blue-Winged Olives) Mayfly Dun View 14 PicturesThis dun molted most of the way into a spinner (though the wings got stuck) the evening after I photographed it, so I took some more photos of the spinner.

I found a female nearby, probably of the same species.
Collected September 19, 2006 from Mystery Creek #43 in New York
Added to Troutnut.com by Troutnut on October 4, 2006
Male Hexagenia limbata (Hex) Mayfly SpinnerMale Hexagenia limbata (Hex) Mayfly Spinner View 13 Pictures
Collected June 26, 2005 from Atkins Lake in Wisconsin
Added to Troutnut.com by Troutnut on May 26, 2006
Specimen Page:1234...70

42 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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A thick mating swarm of Tricorythodes mayfly spinners hovers the West Branch of the Delaware near Hale Eddy one early fall morning. View the picture full-size and you'll be able to make out the wings and tails on most of those little white dots. This was one of many such clouds visible all up and down the river. The mayflies were impressive, but the trout did not hold up their end of the bargain -- there was not a rise in sight. In this picture: Mayfly Genus Tricorythodes (Tricos). From the West Branch of the Delaware River in New York.
A thick mating swarm of Tricorythodes mayfly spinners hovers the West Branch of the Delaware near Hale Eddy one early fall morning. View the picture full-size and you'll be able to make out the wings and tails on most of those little white dots.

This was one of many such clouds visible all up and down the river. The mayflies were impressive, but the trout did not hold up their end of the bargain -- there was not a rise in sight.

In this picture: Mayfly Genus Tricorythodes (Tricos).
StateNew York
LocationWest Branch of the Delaware River
Date TakenSep 2, 2005
Date AddedFeb 8, 2006
AuthorTroutnut
CameraCanon EOS 20D
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Shed exoskeleton from what was very likely an Ephemerella aurivillii nymph that emerged on this rock. In this picture: Mayfly Species Ephemerella aurivillii. From Mystery Creek # 170 in Alaska.
Shed exoskeleton from what was very likely an Ephemerella aurivillii nymph that emerged on this rock.

In this picture: Mayfly Species Ephemerella aurivillii.
StateAlaska
Date TakenJul 11, 2012
Date AddedJul 14, 2012
AuthorTroutnut
CameraCanon PowerShot D10
Streamside Photo Page:12345

67 Underwater Pictures of Mayflies:

Underwater Photo Page:1234...8
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A crayfish chews on a Hexagenia limbata nymph shortly after a small Hex emergence. I didn't catch any fish, but playing around with my flashlight and camera in the rocks proved productive. In this picture: Arthropod Order Decapoda (Crayfish) and Mayfly Species Hexagenia limbata (Hex). From the Namekagon River in Wisconsin.
A crayfish chews on a Hexagenia limbata nymph shortly after a small Hex emergence. I didn't catch any fish, but playing around with my flashlight and camera in the rocks proved productive.

In this picture: Arthropod Order Decapoda (Crayfish) and Mayfly Species Hexagenia limbata (Hex).
StateWisconsin
LocationNamekagon River
Date TakenJun 14, 2006
Date AddedJun 30, 2006
AuthorTroutnut
CameraPENTAX Optio WPi
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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: True Fly Family Simuliidae (Black Flies) and Mayfly Family Baetidae (Blue-Winged Olives). 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: True Fly Family Simuliidae (Black Flies) and Mayfly Family Baetidae (Blue-Winged Olives).
StateWisconsin
LocationEighteenmile Creek
Date TakenMar 19, 2004
Date AddedJan 25, 2006
AuthorTroutnut
CameraOlympus C740UZ
Underwater Photo Page:1234...8

Recent Discussions of Ephemeroptera

Polymitarcyidae Question 1 Reply »
Posted by Steamntrout on Jun 22, 2017 in the species Ephoron album
Last reply on Jun 23, 2017 by Crepuscular
What size are these nymphs & Sub Imago's?
ReplySteamntrout
Posted by Steamntrout on Jun 14, 2017 in the species Leptophlebia nebulosa
Looking at Purdue's May Fly Central it shows nebulosa being found in Canada's Far North, North East and North West as well as USA's North East, South East & Far West.
ReplyClinger nymphs are shaped that way to hang in really fast currents. Really? 14 Replies »
Posted by Entoman on Feb 24, 2013 in the genus Rhithrogena
Last reply on Jun 14, 2017 by Steamntrout
It is commonly held that clingers are flattened to make their lives better adapted to faster water. Their teardrop shape is certainly a classic symbol of aero and hydrodynamic perfection, so there must be some connection, right? It seems to me that such ideas show a complete misunderstanding of the hydraulic reality in which they live. Current is negligable even in the fastest a few mm. from the surface of solid objects. In fact, it is actually quite calm. I've observed baetids clinging by their tippy toes to the tops of rocks in fast riffles with no apparent effort, often next to clinger species that look like they're hanging on for dear life. What if clinger nymphs are flattened not to hold their place in fast currents but rather to facilitate movement in their ecological niche of the cramped spaces under and between cobble or crevices in other substrate types?

It is also thought that the gills of some species form ''suction'' to hold them in place. Since suction is a phenomena of vacuum creation in the atmosphere, how are these nymphs accomplishing this underwater? Is it their ultra delicate gills that hold them in place or a firm claw grip? The horizontal sprawl of the legs masks this as the gills stay in place until the legs brake free. Exposed to the air, the gills seem to laminate against the rock, just as crepe paper would if first held underwater before a rock was lifted out into the air from underneath it. However, underwater their gills behave like the crepe, flowing freely. They are performing their function as gills not suction cups. I find it hard to believe they evolved the way some think merely so they can make it more difficult for humans to pluck them from rocks in the atmosphere. How is it these mighty structures that defy our attempts to pry them from the rocks curiously fall off so easily when prodded for inspection a few seconds later in a tray or jostled in a container on the way home?

Even many scientific papers have encouraged these dubious beliefs so it's not just angler myth... And they go unchallenged... Thoughts?
ReplyHex hatch water temperature range? 4 Replies »
Posted by NEMatt on May 23, 2014 in the species Hexagenia limbata
Last reply on Jul 5, 2016 by Bombillo
Hi,

New to the site - love it. I was wondering if there was a suggested range of water temperature at which the Hex likes to hatch.

Thanks
Matt
ReplyWhat is the big DEAL about the HEX? 21 Replies »
Posted by Spinner on Jun 21, 2006 in the species Hexagenia limbata
Last reply on Jul 5, 2016 by Bombillo
fishing in the dark.......
stepping in holes?

I hate the dark........

I don't need the hex........

Len
Reply
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