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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 Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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 Genera of Glossosomatidae Larvae, Couplet 1

Identification: Key to Genera of Glossosomatidae Larvae, Couplet 1

Adapted from Merritt R.W., Cummins, K.W., and Berg, M.B. (2019)
This couplet refers figures (usually designated 'sf') from this source.
You will need the source (make sure to get the same edition!) to see them.
Option 1Option 2
Mesonotum with 2 or 3 sclerites (sf 19.156, 19.157, sometimes hard to see)Mesonotum without sclerites
Head with ventromesal margins of genae not thickenedHead with ventromesal margins of genae thickened
Posterior median ventral ecdysial line about 1.5 times as long as each anterior divergent branch (sf 19.159, 19.160)Posterior median ventral ecdysial line about as long as each anterior divergent branch (sf 19.158)
Anal opening without dark, sclerotized line on each sideAnal opening with dark, sclerotized line on each side (sf 19.60, 19.164)
Remaining genera: Agapetus, Culoptila, Padunia, and ProtoptilaRemaining genera: Anagapetus and Glossosoma
1 Example Specimen
Go to Couplet 2 Go to Couplet 5
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:
  • Mesonotum with 2 or 3 sclerites (sf 19.156, 19.157, sometimes hard to see)
  • Head with ventromesal margins of genae not thickened
  • Posterior median ventral ecdysial line about 1.5 times as long as each anterior divergent branch (sf 19.159, 19.160)
  • Anal opening without dark, sclerotized line on each side
Couplet 2
Leads to Couplet 5:
  • Mesonotum without sclerites
  • Head with ventromesal margins of genae thickened
  • Posterior median ventral ecdysial line about as long as each anterior divergent branch (sf 19.158)
  • Anal opening with dark, sclerotized line on each side (sf 19.60, 19.164)
Couplet 5
Leads to Couplet 3:
  • Mesonotum with 3 sclerites (sf 19.157)
  • Ventral apotome of head slender laterally, parallel-sided (sf 19.160)
Couplet 3
Leads to Agapetus:
  • Mesonotum with 2 sclerites (sf 19.156)
  • Ventral apotome of head not as slender, broader laterally than mesally (sf 19.159)
Leads to Padunia
(Padunia jeanae):
  • Each tarsal claw apparently trifid, with 3 points subequal in length (sf 19.161)
  • Southeast
Padunia
(Padunia jeanae)
Leads to Couplet 4:
  • Each tarsal claw with normal single point, basal seta and basal process much smaller (sf 19.162, 19.163)
Couplet 4
Leads to Protoptila:
  • Basal seta of each tarsal claw long, thin, and arising from side of stout basal process (sf 19.163)
  • Case constructed with some relatively large stones (sf 19.167)
  • Widespread
Leads to Culoptila:
  • Basal seta of each tarsal claw short and stout, larger than basal process (sf 19.162)
  • Case constructed of uniformly small stones (sf 19.168)
  • Northwest, West
Leads to Glossosoma:
  • Pronotum excised 1/3rd anterolaterally to accommodate coxae (sf 19.165)
  • Dome-like case of stones with transverse ventral strap of finer sand and interchangeable ends (sf 19.38, 19.459a, 19.459b)
  • Widespread
Leads to Anagapetus:
  • Pronotum excised 2/3rds anterolaterally to accommodate coxae (sf 19.166)
  • Case similar to that at left
  • West
Anagapetus

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