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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 jacobi

Species Range


To determine whether a specimen of Caudatella belongs to Caudatella jacobi, 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.

Wing length: 10 mm

Head deep brown; eyes (dried) blackish with slight reddish tinge. Pronotum deep brown, the lateral edges pale yellowish; mesonotum deep brown, shaded with paler laterally and posteriorly, latero-anterior edge deep blackish; scutellum long, narrow, with palish area at base and the thin ends of the axillary cords projecting caudad beyond its apex for a considerable length (as in tibialis and hystrix). Pleura and sternum blackish, shaded with paler around bases of legs and with a large pale yellowish area cephalad of wing-base, crossed by a black line. Metathorax and abdomen blackish, the posterior edges of abdomen paler brown, giving a slight ringed appearance. Forelegs with femora and tibiae pale smoky, the tibia long and almost twice the length of femur; tarsi dull ochreous, paler than the preceding joints. Midand hindlegs dull ochreous, the femora considerably shaded with smoky. Wings narrow, hyaline, with entirely pale and inconspicuous crossveins; longitudinal veins fine and pale brownish in costal half of wing. Tails broken. Length of forewing 10 mm. (After McDunnough 1939.)

Body length: 6 mm
Wing length: 10 mm

Head light brown, eyes blackish. Thorax clay brown, yellowish patch anterior to base of forewing, containing a black line that descends to the coxa of the foreleg; lateral sutures marked with black; wings hyaline, veins light brown; legs dull ochreous. Abdominal terga black with posterior light margins, posterior two segments pale with lateral dark areas; sterna black, posterior two segments with pale lateral margins (caudal filaments missing in described specimen).


Described in Allen, R.K., and Edmunds, George F. Jr. (1961) as Ephemerella orestes
Body length: 6–7 mm

Cerci 8-9 mm; terminal filament 10-11 mm. Head brown without occipital tubercles; antennae brown; maxillary palpi three-segmented. Pronotum and mesonotum brown without tubercles and with irregular light brown markings. Legs brown with rows of long hairs and a few short spines similar to those in fig. 15; tarsal claws with 6-8 denticles. Abdominal terga brown, terga 1-7 dark brown laterally, lighter medially, light areas increase in width posteriorly so that terga 8-10 are nearly light brown as in fig. 24; conspicuous, paired dorsal submedian tubercles on segments 2-9, as in fig. 24, tubercles are narrowly separated on tergum 2 and become more widely separated to tergum 7; tubercles are slightly closer together on terga 8 and 9; sterna dark brown with light brown lateral margins as in fig. 9, anterolateral corners may be darkened. Caudal filaments brown, lateral cerci are 75%-85% as long as terminal filament.

Start a Discussion of Caudatella jacobi


  • 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.

Mayfly Species Caudatella jacobi

Species Range
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