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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 Grammotaulius betteni (Limnephilidae) (Northern Caddisfly) Caddisfly Larva from the Yakima River in Washington
This is a striking caddis larva with an interesting color pattern on the head. Here are some characteristics I was able to see under the microscope, but could not easily expose for a picture:
- The prosternal horn is present.
- The mandible is clearly toothed, not formed into a uniform scraper blade.
- The seems to be only 2 major setae on the ventral edge of the hind femur.
- Chloride epithelia seem to be absent from the dorsal side of any abdominal segments.
Based on these characteristics and the ones more easily visible from the pictures, this seems to be Grammotaulius. The key's description of the case is spot-on: "Case cylindrical, made of longitudinally arranged sedge or similar leaves," as is the description of the markings on the head, "Dorsum of head light brownish yellow with numerous discrete, small, dark spots." The spot pattern on the head is a very good match to figure 19.312 of Merritt R.W., Cummins, K.W., and Berg, M.B. (2019). The species ID is based on Grammotaulius betteni being the only species of this genus known in Washington state.
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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Posts: 2
Wwelz on Jun 26, 2008June 26th, 2008, 2:49 am EDT
I was fishing the Natchaug river a steam in eastern ct on June 23 from five pm till dark. I saw a couple of sulphurs size 18 on the water at six pm. As the evening went on the fish were acting like they were chasing nymphs to the surface sometimes leaping totally out of the water. They were also swirling for something right under the surface. I caught a few fish on a sulphur dry fly but lots of refusals. I never saw a dun emerge after six pm or any sign of any insect emerging. I tried many emerger patterns under the surface with no luck. whats going on here? Behavioral drift?
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Posts: 1681
GONZO on Jun 26, 2008June 26th, 2008, 6:43 am EDT

What you describe sounds very much like what happens when a strong caddisfly emergence coincides with a minor emergence of mayflies. Here are the italicized headings Gary LaFontaine uses in the section "Learning to Recognize a Caddisfly Emergence" in Caddisflies:

One, a trout occasionally leaps into the air.
Two, most of the feeding trout are bulging or splashing.
Three. There are no insects on the surface.

Although these signs do not apply to all caddisfly emergences (some do not emerge on the surface), and other activity could also be the cause, it is a good possibility that this is what you witnessed. Unlike many mayfly emergences, adult caddisflies often are not as obvious on the surface or in the air during an emergence. The times when we see large numbers of caddisflies flying about are usually during mating flights. Unless these coincide with an emergence, they typically have little to do with the feeding activity of trout until the females start laying eggs. (There are a few exceptions to this. For example, I often see trout leaping for the tightly clustered adults in black dancer [Mystacides] mating swarms.)

What I usually do when confronted with the situation you describe is to hold a pocket nymph seine at the surface in a strong line of drift. Sometimes emerging caddisflies are captured, but even when they are not, the filmy, transparent shucks left by emerging caddisflies often show up in the net.
Posts: 2
Wwelz on Jun 26, 2008June 26th, 2008, 9:54 am EDT
Thank you - I think you are right . I will try your suggestions

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