Adult Mayfly Identification

Text: Roger Rohrbeck

Photos: Jason Neuswanger

Many fly fishers are able to recognize adult Mayflies as such, but they may be hard-pressed to go much further with the process of identification.  So, lets explore some easily observable features.  These features can be used to identify a winged mayfly as to dun or spinner.  They can identify it as to male or female.  And often, they even allow identification to genus level.

For those unfamiliar with genus names, a common name for each genus is provided at the end of this article.

After a bit, well walk through the identification of a mayfly, but first we need to set the stage by exploring some physical features of the winged stages of mayflies.

Is it a dun or spinner?

Mayflies are unique in the insect world, in that they have more than one winged lifestage, the dun (sub-adult or subimago), and the spinner (adult or imago).

Under most circumstances, the (dun or spinner) determination is easy to make. 

A mayfly dun (subimago) has wings that are dull and opaque. 

A mayfly spinner (imago) has wings that are hyaline (shiny and transparent).

Is it male or female?

Three easily observable features can be used to determine sex in the winged stages of mayflies:

  • Eyes of male tend to dominate the head; eyes of female dont.
  • Fore legs of male are extremely long; fore legs of female arent.
  • Male has forceps (claspers) to hold female during copulation; female doesnt.

While all three of these features will not necessarily be observable, usually one or two are, and can be used to determine the sex of a winged mayfly.

How long is it?

The body of a mayfly is measured from the front of the head to the end of the abdomen, excluding tails.  This measurement varies greatly from one type of mayfly to another.  It ranges (from 2 mm.) for the smallest species (to 40 mm.) for the largest species.

These are the body length ranges (excluding tails) for most N. American mayfly genera:

Acentrella (4-6 mm.), Ameletus (9-14 mm.), Anthopotamus (7-18 mm.), Asioplax (3-6 mm.), Attenella (6-8 mm.), Baetis (4-10 mm.), Baetisca (10-14 mm.), Brachycercus (3-6 mm.), Caenis (2-4 mm.), Callibaetis (5-10 mm.), Caudatella (8-11 mm.), Centroptilum (5-7 mm.), Cinygma (8-10 mm.), Cinygmula (6-10 mm.) Diphetor (5-7 mm.), Drunella (6-16 mm.), Epeorus (7-14 mm.), Ephemera (10-30 mm.), Ephemerella (5-12 mm.), Ephoron (9-14 mm.), Heptagenia (8-14 mm.), Heterocloeon (5-6 mm.), Hexagenia (16-40 mm.), Ironodes (11-14 mm.), Isonychia (10-21 mm.), Leptophlebia (8-14 mm.), Leucrocuta (5-8 mm.), Litobrancha (15-40 mm.), Maccaffertium (7-16mm.), Metretopus (10 mm.), Nixe (8-11mm.), Paraleptophlebia (6-11 mm.), Plauditus (3-6 mm.), Procloeon (3-9 mm.), Pseudocloeon (4-6 mm.), Rhithrogena (7-11mm.), Serratella (4-9 mm.), Siphlonurus (9-16 mm.), Siphloplecton (13-19 mm.), Stenacron (8-12 mm.), Stenonema (9-11 mm.), Timpanoga (14-16 mm.), Tricorythodes (3-7mm.)

Are wings plain, or are they marked or mottled?

Some mayflies have wings that are plain (uniform in color), and others have wings that are marked or mottled (contrasting in color).

Plain wing

Among mayflies with plain fore wings are the following genera:

Acentrella, Ameletus, Anthopotamus, Asioplax, Attenella, Baetis, Baetisca, Brachycercus, Caenis, Caudatella, Centroptilum, Cinygma, Cinygmula, Diphetor, Drunella, Epeorus, Ephemerella, Ephoron, Heterocloeon, Hexagenia, Ironodes, Isonychia, Leptophlebia, Metretopus, Paraleptophlebia, Plauditus, Procloeon, Pseudocloeon, Serratella, Siphlonurus, Timpanoga, and Tricorythodes.

Marked or mottled wing

Among mayflies with marked or mottled fore wings are the following genera:

Callibaetis, Ephemera, Heptagenia, Leucrocuta, Litobrancha, Maccaffertium, Nixe, Rhithrogena, Siphloplecton, Stenacron, and Stenonema.

What size are the hind wings?

All adult mayflies have fore wings, and most adult mayflies also have hind wings.  Whether the hind wings are obvious, minute, or absent is a key to identification.

Obvious hind wing

Among those mayflies with obvious hind wings are the following genera:

Ameletus, Anthopotamus, Attenella, Baetisca, Caudatella, Cinygma, Cinygmula, Drunella, Epeorus, Ephemera, Ephemerella, Ephoron, Heptagenia, Hexagenia, Ironodes, Isonychia, Leptophlebia, Leucrocuta, Litobrancha, Maccaffertium, Metretopus, Nixe, Paraleptophlebia, Rhithrogena, Serratella, Siphlonurus, Siphloplecton, Stenacron, Stenonema, and Timpanoga.

Minute hind wing

Among those mayflies with minute hind wings are the following genera:

Acentrella, Baetis, Callibaetis, Centroptilum, Diphetor, and Heterocloeon.

Absent hind wing

Among the mayflies with absent (no) hind wings are the following genera:

Asioplax, Brachycercus, Caenis, Plauditus, Procloeon, Pseudocloeon, and Tricorythodes.

Does it have 2 or 3 tails?

Winged mayflies have either two or three tails.  Dont confuse this with the number of tails the nymph has, as it may be different.  Some mayflies have three tails as a nymph, but only two in the winged stages.

2-tailed Families (Genera)

Ameletidae (Ameletus)
Baetidae (Acentrella, Baetis, Callibaetis, Centroptilum, Diphetor, Heterocloeon, Plauditus, Procloeon, Pseudocloeon)
Ephemeridae (Hexagenia, Litobrancha)
Heptageniidae (Cinygma, Cinygmula, Epeorus, Heptagenia, Ironodes, Leucrocuta, Maccaffertium, Nixe, Rhithrogena, Stenacron, Stenonema)
Isonychiidae (Isonychia)
Metretopodidae (Metretopus, Siphloplecton)
Polymitarcyidae (Ephoron male)
Siphlonuridae (Siphlonurus)

3-tailed Families (Genera)

Baetiscidae (Baetisca)
Caenidae (Brachycercus, Caenis)
Ephemeridae (Ephemera)
Ephemerellidae (Attenella, Caudatella, Drunella, Ephemerella, Serratella, Timpanoga)
Leptohyphidae (Asioplax, Tricorythodes)
Leptophlebiidae (Leptophlebia, Paraleptophlebia)
Polymitarcyidae (Ephoron female)
Potamanthidae (Anthopotamus)

First, lets "Observe"

Okay, now that weve explored some physical attributes of the winged stages of mayflies, lets put your new observation skills to use.  Well assume youve just parked your vehicle, and are in the process of putting on your waders.  At this very moment, a mayfly lands on the back of your hand.  You gently lift your arm, and make some observations before the mayfly discovers its mistake, and flies off.

  • It looks to be 9 mm. (~3/8) in length, about hook size #14.
  • The eyes dominate the head.
  • The fore legs are extremely long.
  • You arent sure whether or not it has claspers.
  • The fore wings are dark in color and marked with broad white veins.
  • The hind wings are minute (luckily, youre blessed with 20/10 eyesight).
  • It has two tails.

Then, some "analysis"

Your new friend didnt have hyaline wings, so it must be a dun.  Its eyes dominated the head, and it had extremely long fore legs, so it must be a male.

By consulting the number of tails lists (see above), we can eliminate all but the following genera, which have a length range including 9 mm.

Acentrella, Ameletus, Baetis, Callibaetis, Centroptilum, Cinygma, Cinygmula, Diphetor, Epeorus, Ephemerella, Ephoron (male), Heptagenia, Heterocloeon, Ironodes, Isonychia, Leucrocuta, Litobrancha, Maccaffertium, Metretopus, Nixe, Plauditus, Procloeon, Pseudocloeon, Rhithrogena, Siphlonurus, Siphloplecton, Stenacron, and Stenonema.

By consulting the body length range list, we are able to eliminate all from the above list except:

Ameletus, Baetis, Callibaetis, Cinygma, Cinygmula, Epeorus, Ephoron (male), Heptagenia, Maccaffertium, Nixe, Procloeon, Rhithrogena, Siphlonurus, Stenacron, and Stenonema.

By consulting the fore wings lists, we are able to eliminate all from the above list except:

Callibaetis, Heptagenia, Maccaffertium, Nixe, Rhithrogena, Stenacron, and Stenonema.

By consulting the hind wings lists, we are able to eliminate all from the above list except:

Callibaetis.

So, as a result of this observation and analysis, you astutely conclude that your new friend must have been a Callibaetis male dun, commonly called a Specklewing. 

Too much work?

My goodness (you say), this analysis is way too much work.  Do I really have to do all that just to identify a winged mayfly I dont already recognize?  For the answer, see paragraph below.

Author and photographer

Roger Rohrbeck, an entomology enthusiast from the state of Washington, provided the text for this article.  The above labor-intensive analysis is automatically performed by the Identification page on his website, http://www.flyfishingentomology.com/. One need only enter their observations, and click the Lookup button.

Jason Neuswanger generously authorized use of his insect photos for this article.  In most cases, the photos have been cropped, annotated, or otherwise altered for illustration purposes.  Jasons vast gallery of aquatic insect photos can be viewed on his highly acclaimed website, http://www.troutnut.com/.

Common names for genera

Acentrella (Minute Graywinged Brown), Ameletus (Brown Dun), Anthopotamus (Golden Drake), Asioplax (Tiny Whitewinged Olive Quill), Attenella (Golden Drake), Baetis (Bluewinged Olive), Baetisca (Specklewinged Dun), Caenis (Whitewinged Sulphur), Callibaetis (Specklewing), Caudatella (no common name), Centroptilum (Pale Watery Dun), Cinygma (Western Light Cahill), Cinygmula (Dark Red Quill), Diphetor (Iron Blue Quill), Drunella (Green Drake), Epeorus (Slate Brown Dun), Ephemera (Brown Drake), Ephemerella (Pale Morning Dun), Ephoron (White Drake), Heptagenia (Pale Evening Dun), Heterocloeon (Bluewinged Olive), Hexagenia (Hex), Ironodes (Slate Maroon Drake), Isonychia (Leadwing), Leptophlebia (Black Quill), Leucrocuta (Pale Evening Dun), Litobrancha (Dark Green Drake), Maccaffertium (Light Cahill), Metretopus (no common name), Nixe (Western Ginger Quill), Paraleptophlebia (Mahogany Dun), Plauditus (Bluewinged Olive), Procloeon (Bluewinged Olive), Pseudocloeon (Bluewinged Olive), Rhithrogena (Western March Brown), Serratella (Little Dark Hendrickson), Siphlonurus (Gray Drake), Siphloplecton (Speckled Olive), Stenacron (Cahill), Stenonema (Gray Fox), Timpanoga (Great Bluewinged Red Quill), Tricorythodes (Trico).

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