Blue-winged Olives
Baetis
Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
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This specimen keys pretty easily to Onocosmoecus, and it closely resembles a specimen from Alaska which caddis expert Dave Ruiter recognized as this genus. As with that specimen, the only species in the genus documented in this area is Onocosmoecus unicolor, but Dave suggested for that specimen that there might be multiple not-yet-distinguished species under the unicolor umbrella and it would be best to stick with the genus-level ID. I'm doing the same for this one.
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Identification: Easy Angler's Key to Common Adult Mayflies, Couplet 6
Identification: Easy Angler's Key to Common Adult Mayflies, Couplet 6
| Option 1 | Option 2 |
|---|---|
Wings usually with irregular, blotchy marks 
| Wings of dun usually shades of uniform gray; wings of spinner usually clear 
|
| Very large bodies typically at least 15 mm long (hook size 8 or larger) | Medium-large to small bodies typically less than 15 mm long (hook size 10 or smaller), although the largest species can just reach hook size 8 |
Body with a very elongated abdomen, almost serpentine; the "drake" shape characteristic of their long-bodied burrowing nymphs 
| Body with more "average" mayfly proportions, not exceptionally elongated 
|
Hind wing large, typically one third to one half the length of forewing 
| Hind wing size variable, but typically about one quarter to one third the length of forewing 
|
| Remaining families: Ephemerellidae and Leptophlebiidae | |
5 Example Specimens
For years after I started this website, I was eagerly hoping to find a green drake to add to the collection, but I was never in the right part of the world at the right time. It finally happened on June 1st, 2007.
This spinner was the only member of its species I saw all night during an incredibly thick and tricky mixed hatch on Penn's Creek a few days before the real start of its famous green drake hatch.
This yellow drake dun hatched out of my aquarium over a month before her brethren in the wild are slated to emerge. She seems a bit small, and that might be the reason.
| 5 Example Specimens
This dun is smaller than would normally be expected for Drunella flavilinea, but it seems to fit the physical description of that species fairly well. I wasn't sure from the dun pictures alone, so I was hoping it would molt into a spinner. It never made it out of the dun's shuck, but it did begin the process, which allowed me with great care to tease out the last couple abdominal segments of the spinner from the still tightly attached dun shuck, enabling a more confident ID based on the shape of the spinner's penes and sub-genital plate. I also collected a nymph of flavilinea in the same river on the same evening, as well as a shed exuvium that looked large enough to belong to an emerging dun.
The other possibility I considered for a while based on the dun was Drunella pelosa, which would be the right size, but the other characteristics don't fit. 
The positive species ID on this dun comes from both the spinner that it (or possibly one other dun just like it) molted into and the overwhelming abundance of nymphs of this species in my kicknet samples from the same site.
This dun hatched in my aquarium on July 16th from an easily identified nymph collected on July 10th, and it molted into a spinner after I photographed it. The beautiful spinner form is listed as separate specimen. I forgot to photograph the dun with the ruler, but naturally his size is pretty similar to what it was as a spinner.
A few days after unexpectedly collecting a female dun of this species, but in a sad, bedraggled shape, I went back to the same pool to see if I could find more. I was able to catch this one male dun during a dusk emergence dominated by other species, and he made it home in prime shape for photos.
I'm fairly confident calling this dun Ephemerella tibialis because it emerged from one of several similar nymphs in my bucket, which I was able to ID with quite a bit of certainty. However, it looks very different from the other duns I currently have listed, which makes me rethink their IDs.
|
| Ephemeridae Genus Ephemera | Go to Couplet 7 |
The current couplet is highlighted with darker colors and a icon, and couplets leading to this point have a icon.
Leads to Couplet 5:
- Hind wings absent
- Body length 2–7 mm (hook sizes 20 & smaller)
Couplet 5
Leads to Couplet 6:
- Hind wings present
- Body length usually > 7 mm (hook sizes 18 & larger)
Couplet 6
Leads to Baetiscidae
(Baetisca):
(Baetisca):
- Abdomen mostly very thick, then tapering off rapidly in the last 4 segments
- Hind wing rounded, almost circular
- Wings of dun blotchy, wings of spinner clear
- Body length 7–10 mm (hook sizes 14–18)
- Fairly uncommon
Leads to Couplet 8:
- Abdomen thinner in proportion to the rest of the body; more "normal" mayfly shape
- Hind wing more oblong and often less rounded, more angular
- Wings variable
- Length variable
- Very common
Couplet 8
Leads to Leptohyphidae
(Tricorythodes):
(Tricorythodes):
- Abdomen black, dark brown, or olive green
- Wings without a black stripe on the leading edge, although thick veins there may be darker than the rest of the wing
- Male tails around twice the length of the body
- Very common on many trout streams
Couplet 6 (You are here)
Leads to Ephemeridae
(Ephemera):
(Ephemera):
- Wings usually with irregular, blotchy marks
- Very large bodies typically at least 15 mm long (hook size 8 or larger)
- Body with a very elongated abdomen, almost serpentine; the "drake" shape characteristic of their long-bodied burrowing nymphs
- Hind wing large, typically one third to one half the length of forewing
Leads to Couplet 7:
- Wings of dun usually shades of uniform gray; wings of spinner usually clear
- Medium-large to small bodies typically less than 15 mm long (hook size 10 or smaller), although the largest species can just reach hook size 8
- Body with more "average" mayfly proportions, not exceptionally elongated
- Hind wing size variable, but typically about one quarter to one third the length of forewing
Couplet 7
Leads to Ephemerellidae:
- Hind wings with a distinct costal angulation, i.e. a small, rounded bump in the front edge
Leads to Leptophlebiidae:
- Hind wings without costal angulation, except in some species rarely found on trout streams
- Continue to Couplet 2 of the Key to Genera of Male Leptophlebiidae Spinners to easily narrow it down one step further.
Leads to Polymitarcyidae
(Ephoron):
(Ephoron):
- All legs of female, and all except front legs of male, feeble and non-functional
Leads to Couplet 9:
- Legs normal and fully functional
Couplet 9
Leads to Isonychiidae
(Isonychia):
(Isonychia):
- Forelegs dark; middle and rear legs pale
Leads to Couplet 10:
- All legs roughly similar to each other in color
Couplet 10
Leads to Couplet 11:
- Intercalary veins between vein CuA and CuP in the forewing consist of a series of small veinlets, often forking or curved, that connect vein CuA directly to the hind margin of the wing
Couplet 11
Leads to Couplet 12:
- Intercalary veins in this region variable, but not as at left
Couplet 12
Leads to Siphlonuridae
(Siphlonurus):
(Siphlonurus):
- Eyes of male with a pale, horizontal band
- Hind wings with a rounded costal angulation
Leads to Ameletidae
(Ameletus):
(Ameletus):
- Eyes of male without a pale, horizontal band
- Hind wings with a pointed costal angulation
Leads to Ephemeridae
(Hexagenia):
(Hexagenia):
- The largest North American mayflies, with typical body lengths greater than 17 mm
- Body with a very elongated abdomen, almost serpentine; the "drake" shape characteristic of their long-bodied burrowing nymphs
- Hind wing large, typically almost half the length of forewing
Leads to Heptageniidae:
- Body lengths very from small to medium-large, but almost always less than 17 mm
- Body proportions may have a very "average" mayfly shape Alternatively, in many cases, the head, legs, or body might appear somewhat flattened and splayed-out, reflecting the flattened profile of their clinger-type nymphs
- Hind wing size variable, typically around one quarter to one third the length of forewing
Start a Discussion of this Couplet
Families excluded
These families of Ephemeroptera are not included in this key: Metretopodidae, Behningiidae, Neoephemeridae, Potamanthidae, Acanthametropodidae, Ametropodidae, Arthropleidae, Oligoneuriidae, Pseudironidae, Euthyplociidae, Palingeniidae
Description of this key
This key is helps identify common adult mayflies using characteristics that are relatively easy to see and interpret. It omits many rare mayflies and uses characteristics that aren't all 100 % reliable, but it should put anglers close to the right answer most of the time. However, when accuracy is critical, you should use the full Key to Families of Mayfly Duns and Spinners.