For those unfamiliar with genus names, a common name for each genus is provided at the end of this article.
After a bit, we’ll 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.
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).
Three easily observable features can be used to determine sex in the winged stages of mayflies:
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.
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.)
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.
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.
Winged mayflies have either two or three tails. Don’t 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)
Okay, now that we’ve explored some physical attributes of the winged stages of mayflies, let’s put your new observation skills to use. We’ll assume you’ve 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.
Your new friend didn’t 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.
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 don’t already recognize? For the answer, see paragraph below.
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. Jason’s vast gallery of aquatic insect photos can be viewed on his highly acclaimed website, http://www.troutnut.com/.
This week’s typing tip is in response to a question by Hatches reader, Nick S. from Boise, ID. Nick wanted to know if we had any suggestions for a small, lightweight fly tying vise to use streamside, or on backcountry fly-in/ hike-in fly fishing trips.
GLOBE PEQUOT ( THE LYONS PRESS, FALCON), November 1997
Binding Type: Hardcover
Retail Price: $16.95 at the Hatches Store
ISBN: 1-55821-067-9
“The trout’s biggest advantage is selectivity, and we can counteract it only by knowing the insects that make up his diet. This is the reason for the study of stream entomology by the angler, and it is often the weak link in his skill.”
-Ernest Schwiebert
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