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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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Mayfly Genus Caenis (Angler's Curses)

When the important hatches of Tricorythodes were first discovered by anglers, Caenis was given the credit. We now know that the Caenis mayflies are a different group, smaller and less common in trout streams, and they hatch in the evening instead of the morning.

They very rarely elicit selective feeding, but when they do they're very tough to match because they're often much smaller than size 28. This difficulty has earned them the nickname "Angler's Curse."

Where & when

Time of year : June through early September; best in June and July

Preferred waters: Rivers and lakes

Most Caenis mayflies emerge in the evenings when other, larger mayflies are abundant on the water. This limits their importance.

In 320 records from GBIF, adults of this genus have mostly been collected during July (28%), June (26%), August (24%), May (10%), and September (8%).

In 145 records from GBIF, this genus has been collected at elevations ranging from 26 to 8022 ft, with an average (median) of 2690 ft.

Genus Range

Hatching behavior

Caenis mayflies typically emerge, molt into spinners, mate, and oviposit within one hour.

Nymph biology

Substrate: Silt, weeds

Every book says pools and stagnant back-waters are the prime habitats of these nymphs. This is probably true, but I have often found them in gravel, vegetation, and other habitats. They have operculate gills adapted to survival on silty bottoms.

Specimens of the Mayfly Genus Caenis

1 Female Dun
1 Female Spinner
9 Nymphs

Start a Discussion of Caenis

References

Mayfly Genus Caenis (Angler's Curses)

Genus Range
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