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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
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.

Dorsal view of a Neoleptophlebia (Leptophlebiidae) Mayfly Nymph from the Yakima River in Washington
Some characteristics from the microscope images for the tentative species id: The postero-lateral projections are found only on segment 9, not segment 8. Based on the key in Jacobus et al. (2014), it appears to key to Neoleptophlebia adoptiva or Neoleptophlebia heteronea, same as this specimen with pretty different abdominal markings. However, distinguishing between those calls for comparing the lengths of the second and third segment of the labial palp, and this one (like the other one) only seems to have two segments. So I'm stuck on them both. It's likely that the fact that they're immature nymphs stymies identification in some important way.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
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Identification: Easy Angler's Key to Common Adult Mayflies

Identification: Easy Angler's Key to Common Adult Mayflies

Option 1Option 2
Two tails Three tails
Remaining families: Ameletidae, Baetidae, Baetiscidae, Ephemeridae, Heptageniidae, Isonychiidae, Polymitarcyidae, and SiphlonuridaeRemaining families: Caenidae, Ephemerellidae, Ephemeridae, Leptohyphidae, and Leptophlebiidae
5 Example Specimens
5 Example Specimens
Go to Couplet 2 Go to Couplet 3
The current couplet is highlighted with darker colors and a icon, and couplets leading to this point have a icon.
Couplet 1

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Couplet 1 (You are here)
Leads to Couplet 2:
  • Two tails
Couplet 2
Leads to Couplet 3:
  • Three tails
Couplet 3
Leads to Baetidae:
  • Hind wings tiny or absent
Leads to Couplet 4:
  • Hind wings obvious
Couplet 4
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):
  • 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):
  • 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
Leads to Caenidae:
  • Abdomen whitish
  • Wings often with a black stripe on the leading edge
  • Male tails around 5 times the length of the body
  • Less common on most trout streams
Leads to Ephemeridae
(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):
  • 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):
  • 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):
  • Eyes of male with a pale, horizontal band
  • Hind wings with a rounded costal angulation
Leads to Ameletidae
(Ameletus):
  • Eyes of male without a pale, horizontal band
  • Hind wings with a pointed costal angulation
Leads to Ephemeridae
(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

References

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.
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