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.

The "Fountain Flats" stretch of the Firehole was fishable in the morning after an uncharacteristically cold August night.

From the Firehole River in Wyoming

Stream photos

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Ephemerella mucronata Mayfly Nymph Ephemerella mucronata Mayfly Nymph

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.

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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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Hexagenia bilineata?

149113 has attached these 7 pictures to aid in identification. The message is below.

149113
Pennsy

Posts: 3

149113 on Jul 21, 2016July 21st, 2016, 9:45 am EDT

Found these two around Middletown, PA last evening. Both spinners with one spent and one upright. Looking at the previous posts it would seem these are female? These measured 27mm. Previous post seem to indicate Hexagenia bilineata

Taxon

Site Editor
Plano, TX

Posts: 1311

Taxon on Jul 21, 2016July 21st, 2016, 9:42 pm EDT

Hi Bob-

Welcome aboard. Yes, I believe the first two photos to be Hexagenia bilineata female imagoes. I believe the others to be Hexagenia female subimagoes of undetermined species.

Best regards,
Roger Rohrbeck
www.FlyfishingEntomology.com

Crepuscular

Boiling Springs, PA

Posts: 920

Crepuscular on Jul 22, 2016July 22nd, 2016, 7:48 am EDT

Yes sir. Like Roger said that's H bilineata. big girls! Thanks for posting.

Eric

149113
Pennsy

Posts: 3

149113 on Jul 22, 2016July 22nd, 2016, 8:44 am EDT

Thanks guys! I shot a few pics of one last evening and added those to the original post... couldn't figure out how to add them to a separate post. Looking at other ID's these seem like Hexagenia limbata but just wanted to confirm. This specimen measured 24.5mm and was a subimago. It was just hanging out near a light fixture with a lot of smaller mayflies

Jmd123

Oscoda, MI

Posts: 2474

Jmd123 on Jul 22, 2016July 22nd, 2016, 10:58 am EDT

I love anything Hexagenia. The guys who flyfish the Huron River for smallies downstate love this hatch, and this is when they show up, into August too. Didn't get enough of Hex mania in June? Well here's another chance...

Jonathon

No matter how big the one you just caught is, there's always a bigger one out there somewhere...

Crepuscular

Boiling Springs, PA

Posts: 920

Crepuscular on Jul 23, 2016July 23rd, 2016, 2:52 am EDT

Looking at other ID's these seem like Hexagenia limbata but just wanted to confirm. This specimen measured 24.5mm and was a subimago. It was just hanging out near a light fixture with a lot of smaller mayflies

I'd call that H. limbata. And I have collected them together with H. bilineata from the Susquehanna River.

149113
Pennsy

Posts: 3

149113 on Jul 23, 2016July 23rd, 2016, 9:09 am EDT

Looking at other ID's these seem like Hexagenia limbata but just wanted to confirm. This specimen measured 24.5mm and was a subimago. It was just hanging out near a light fixture with a lot of smaller mayflies

I'd call that H. limbata. And I have collected them together with H. bilineata from the Susquehanna River.

Thanks! I saw some of the white flies last evening down by the park on the river. Considerably smaller than these hex mayflies but there were a lot of them