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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 Setvena wahkeena (Perlodidae) (Wahkeena Springfly) Stonefly Nymph from Mystery Creek #199 in Washington
As far as I can tell, this species has only previously been reported from one site in Oregon along the Columbia gorge. However, the key characteristics are fairly unmistakable in all except for one minor detail:
— 4 small yellow spots on frons visible in photos
— Narrow occipital spinule row curves forward (but doesn’t quite meet on stem of ecdysial suture, as it's supposed to in this species)
— Short spinules on anterior margin of front legs
— Short rposterior row of blunt spinules on abdominal tergae, rather than elongated spinules dorsally
I caught several of these mature nymphs in the fishless, tiny headwaters of a creek high in the Wenatchee Mountains.
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 Species Caudatella heterocaudata

This species is quite rare, but Fred Arbona in Mayflies, the Angler, and the Trout says creates "fair" hatches in places.

Where & when

Time of year : July and August

In 1 record from GBIF, this species has been collected at elevation of 1699 ft.

Species Range

Hatching behavior

Time of day : Midday

Nymph biology

Current speed: Fast

Identification

To determine whether a specimen of Caudatella belongs to Caudatella heterocaudata, use the Key to Species of Caudatella Nymphs.

Physical description

Most physical descriptions on Troutnut are direct or slightly edited quotes from the original scientific sources describing or updating the species, although there may be errors in copying them to this website. Such descriptions aren't always definitive, because species often turn out to be more variable than the original describers observed. In some cases, only a single specimen was described! However, they are useful starting points.

Body length: 6–7 mm
Wing length: 6.5–7.5 mm

Eyes reddish-brown. Thorax olivebrown, yellowish patch anterior to the base of the wing, containing a black line which descends to the coxa of the foreleg; lateral sutures marked with black; legs pale olive-brown; fore femora and tibia with black markings; wings hyaline with pale veins and cross veins. Abdominal terga olive-brown, anterior half of each of the first four segments with broad black bands, bands on posterior segments reduced to large semicircular dorsal patches; sterna pale olive-brown with ~midventral and sublateral longitudinal dark brown stripes as in fig. 7. Caudal filaments brown, lateral cerci 20%-30% as long as terminal filament. Penes with a shallow median apical depression as in fig. 3.

Body length: 6–7 mm
Wing length: 6.5–7.5 mm

Similar to male except for usual sexual differences.

Body length: 7–8 mm

Cerci 2-3 mm long; terminal filament 10-12 mm long. Head brown with pale markings, roughened but without occipital tubercles; antennae brown ; maxillary palpi three-segmented as in fig. 12. Thorax brown; legs brown, femora with a few marginal spines and a distinct row of marginal hairs, tibiae and tarsi with a distinct row of marginal hairs (fig. 15), tarsal claws with three to seven denticles. Abdominal terga olive-brown; anterior half of each of the first four segments with broad black bands, bands on posterior segments reduced to large semicircular dorsal patches; each tergum with a series of lateral black patches; paired dorsal abdominal tubercles on segments 1-9 or 2-9 as in figs. 1 and 23; sterna pale olive-brown with median and paired sublateral longitudinal dark brown stripes as in fig. 7. Caudal filaments light brown, darker at the joinings; lateral cerci 20% to 30% as long as terminal filament.

Specimens of the Mayfly Species Caudatella heterocaudata

1 Male Dun
1 Female Dun

Start a Discussion of Caudatella heterocaudata

References

  • Allen, R.K., and Edmunds, George F. Jr. 1961. A Revision of the Genus Ephemerella (Ephemeroptera: Ephemerellidae) II. The Subgenus Caudatella. Annals of the Entomological Society of America 54: 603-612.
  • Arbona, Fred Jr. 1989. Mayflies, the Angler, and the Trout. Nick Lyons Books.

Mayfly Species Caudatella heterocaudata

Species Range
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