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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.

Lateral view of a Male Baetidae (Blue-Winged Olive) Mayfly Dun from Mystery Creek #308 in Washington
This dun emerged from a mature nymph on my desk. Unfortunately its wings didn't perfectly dry out.
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.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

This topic is about Couplet 9:

It is surprisingly difficult to find an easy characteristic to separate Siphlonurus from Callibaetis nymphs, except by size at maturity, and there's some overlap in that. The mouthpart characteristic (notched labrum) in Couplet 16 of the Key to Families of Mayfly Nymphs is definitive, but that's difficult to use without a microscope.
Option 1Option 2
Body relatively large, 9–17 mm when matureBody smaller, 6–10 mm when mature
Found more often in rivers, although usually in slower habitatsFound more often in lakes, although sometimes abundant in slow-moving river habitats
5 Example Specimens
1 Example Specimen
Siphlonuridae Baetidae

This branch only goes to Callibaetis within the broad family Baetidae, most of which lead elsewhere in the key due to their different gill structure.

Troutnut's profile picture
Bellevue, WA

Posts: 2758
Troutnut on May 11, 2023May 11th, 2023, 12:15 am EDT
I can recognize them at a glance, but it's hard to put that into words. In constructing this "easy angler's key," this was one of the hardest distinctions to make without reference to technical characteristics that aren't available to the average person without a microscope and some training and dissection tools. I'm wondering if anybody else knows some good, easy-to-use methods to tell these two apart besides size and slightly different habitat preferences.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist

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