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Lateral view of a Female Hexagenia limbata (Ephemeridae) (Hex) Mayfly Dun from the Namekagon River in Wisconsin
Hex Mayflies
Hexagenia limbata

The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.

Dorsal view of a Epeorus albertae (Heptageniidae) (Pink Lady) Mayfly Nymph from the East Fork Issaquah Creek in Washington
This specimen keys to the Epeorus albertae group of species. Of the five species in that group, the two known in Washington state are Epeorus albertae and Epeorus dulciana. Of the two, albertae has been collected in vastly more locations in Washington than dulciana, suggesting it is far more common. On that basis alone I'm tentatively putting this nymph in albertae, with the large caveat that there's no real information to rule out dulciana.
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: Key to Families of Stonefly Nymphs, Couplet 8

Identification: Key to Families of Stonefly Nymphs, Couplet 8

Option 1Option 2
Apical maxillary palpal segment small, asymmetrically set on penultimate segment
Figure from this Suwallia nymph.
Apical maxillary palpal segment subequal in length to penultimate segment and symmetrically attached
Tails 3/4 or less the length of the abdomen
Figure from this Suwallia nymph.
Tails more than 3/4 the length of the abdomen
Head and thoracic terga usually without a distinct pattern of dark and light pigments Head and thoracic terga usually with a distinct pattern of dark and light pigments
5 Example Specimens
5 Example Specimens
Chloroperlidae Perlodidae
The current couplet is highlighted with darker colors and a icon, and couplets leading to this point have a icon.
Leads to Pteronarcyidae:
  • Ventral gill tufts present on thorax and abdominal segments 1-2 or 1-3
Leads to Couplet 2:
  • Ventral gill tufts absent from abdominal segments 1-2 or 1-3
Couplet 2
Leads to Peltoperlidae:
  • Single, double, or forked conical gills present at least behind coxae of second and third leg
  • Thoracic sterna posteriorly produced, overlapping succeeding segment
  • Some genera appear roach-like
Leads to Couplet 3:
  • Gills, if present, not conical
  • Thoracic sterna not overlapping
Couplet 3
Leads to Couplet 4:
  • Paraglossae and glossae extend forward about the same distance (sf 16.18) (be careful not to confuse them with the labial palps, which might be large and partially obscuring these parts, as in the illustration)
Couplet 4
Leads to Couplet 7:
  • Paraglossae much longer than glossae
Couplet 7
Leads to Taeniopterygidae:
  • First and second tarsal segments near equal in length
  • Midline of wing pads strongly divergent from body axis
  • One genus often with each coxa bearing a single, segmented, ventrally directed, finger-like gill (sf 16.20) or abdomen terminating in a large, ventral, shield-like plate (sf 16.53)
Leads to Couplet 5:
  • Second tarsal segment much shorter than first
  • Midline of wing pads parallel or divergent
  • No coxal finger-like gills nor large ventral, shield-like plate
Couplet 5
Leads to Nemouridae:
  • Midline of metathoracic wing pads strongly divergent from body axis
  • Usually robust and squat body, such that extended hind legs reach about to the tip of the abdomen
  • Cervical gills present (sf 16.22-16.25) or absent
Leads to Couplet 6:
  • Midline of metathoracic wingpads, if present, nearly parallel with body axis (sf 16.11-16.12)
  • Elongate body, such that extended hind legs reach far short of the tip of the abdomen
  • No cervical gills
Couplet 6
Leads to Capniidae:
  • Postmentum small, its anterior portion not covering bases of maxillae (sf 16.26)
  • Terga and sterna of abdominal segments 1-9 separated by membraneous pleural fold (sf 16.28)
  • Abdominal segments widest posteriorly
  • Hind wingpads about as broad as long (sf 16.30-16.31), reduced, or absent
  • Abdominal terga always with a posterior setal fringe (sf 16.32)
Leads to Leuctridae:
  • Postmentum large, with its anterior portion partially covering basis of maxillae (sf 16.27)
  • At most, abdominal segments 1-7 separated by membraneous pleural fold (sf 16.29)
  • Abdominal segments uniformly cylindrical
  • Hind wing pads usually longer than wide (sf 16.12)
  • Abdominal terga of some genera (Despaxia, Perlomyia, Paraleuctra, and Zealuctra) without a posterior setal fringe
Leads to Perlidae:
  • Highly branched filamentous gills on sides and venter of thorax
Leads to Couplet 8:
  • Branched, filamentous gills absent from thorax although sometimes it has a single thumb-like gill (sf 16.34) or rarely forked gills (sf 16.35)
Couplet 8
Couplet 8

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Couplet 8 (You are here)
Leads to Chloroperlidae:
  • Apical maxillary palpal segment small, asymmetrically set on penultimate segment
  • Tails 3/4 or less the length of the abdomen
  • Head and thoracic terga usually without a distinct pattern of dark and light pigments
Leads to Perlodidae:
  • Apical maxillary palpal segment subequal in length to penultimate segment and symmetrically attached
  • Tails more than 3/4 the length of the abdomen
  • Head and thoracic terga usually with a distinct pattern of dark and light pigments

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References

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