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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 Ephemerella mucronata (Ephemerellidae) Mayfly Nymph from the Yakima River in Washington
This is an interesting one. Following the keys in Merritt R.W., Cummins, K.W., and Berg, M.B. (2019) and Jacobus et al. (2014), it keys clearly to Ephemerella. Jacobus et al provide a key to species, but some of the characteristics are tricky to interpret without illustrations. If I didn't make any mistakes, this one keys to Ephemerella mucronata, which has not previously been reported any closer to here than Montana and Alberta. The main character seems to fit well: "Abdominal terga with prominent, paired, subparallel, spiculate ridges." Several illustrations or descriptions of this holarctic species from the US and Europe seem to match, including the body length, tarsal claws and denticles, labial palp, and gill shapes. These sources include including Richard Allen's original description of this species in North America under the now-defunct name E. moffatae in Allen RK (1977) and the figures in this description of the species in Italy.
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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Baetis bicaudatus (BWO) Mayfly Nymph Pictures

Here I'm just copying and pasting, without cleaning up, my notes from spending a long time with this one under the microscope (and keying with Merritt & Cummins 5th Ed) only to end up confirming the most likely guess.

7. Baetis bicaudatus nymph
1. Hind wingpad present but small and hidden beneath forewing pad
2. Segment 2 of labial palp with well-developed medially projecting corner --> Baetis (couple 44). BUT no sign of scale-like setae on abdominal terga. Conflicts at this couplet.
3. Gills on segments I-VII
4. Tarsal claws with denticles, seemingly 2 rows but very hard to tell… and the key options with 2 rows don’t make sense
5. Assuming no villipore, we land confidently at couplet 48
6. Leads to Fallceon, except antennal scape doesn't have robust setae
7. Treated as Baetis, leads to brunneicolor, but McDunnough et al 1932 (Can Ent 64) suggests middle tail should be 5/6 as long as outer ones
8. Keys VERY confidently to couplet 36 in M&C (villipore)
9. If assuming villipore present:
1. 37 --> Scape of antennae has no distal lobe --> rules out Labiobaetis (100 % certain)
2. 38 --> Terminal filament much shorter than cerci --> not Barbaetis benfieldi (100 % certain)
3. 39 --> Terminal filament reduced (100 % certain)
4. 40 --> Tarsal claw denticle count couplet. If two rows of denticles: Either Iswaeon or Heterocloeon. Can't be Iswaeon because cerci lack dark median band. Can't be heterocloeon because it's not in the Platte drainage or in Texas. Thus, it must be one row of denticles. Moving on to 42.
5. 42 --> Hind wing pads present (100 % certain)
6. 44 --> Segment 2 of labial palpi with well-developed medially projecting corner (80 % certain), scale-like setae not evident on terga but maybe limitation of my scope --> Baetis (alternative would be Acentrella, but pronotum shape is all wrong for those, although not an official characteristic)
7. CONFIDENT in Baetis bicaudatus after distinctive leg markings (J-shaped light mark on first femur, L-shaped on second and third) matches original species description to a tee.

Baetis bicaudatus (Baetidae) (BWO) Mayfly Nymph from Green Lake Outlet in Idaho
Baetis bicaudatus (Baetidae) (BWO) Mayfly Nymph from Green Lake Outlet in Idaho
The J-shaped light mark on the first femur and L-shaped marks on the next two are telltale signs of Baetis bicaudatus according to the original species description.

Dorsal view of a Baetis bicaudatus (Baetidae) (BWO) Mayfly Nymph from Green Lake Outlet in Idaho
Ventral view of a Baetis bicaudatus (Baetidae) (BWO) Mayfly Nymph from Green Lake Outlet in Idaho
Ruler view of a Baetis bicaudatus (Baetidae) (BWO) Mayfly Nymph from Green Lake Outlet in Idaho The smallest ruler marks are 1 mm.

This mayfly was collected from Green Lake Outlet in Idaho on August 4th, 2020 and added to Troutnut.com by Troutnut on August 20th, 2020.

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Baetis bicaudatus (BWO) Mayfly Nymph Pictures

Collection details
Location: Green Lake Outlet, Idaho
Date: August 4th, 2020
Added to site: August 20th, 2020
Author: Troutnut
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