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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 Glossosoma (Glossosomatidae) (Little Brown Short-horned Sedge) Caddisfly Larva from the Yakima River in Washington
I caught this tiny larva without a case, but it seems to key pretty clearly to to Glossosomatidae. From there, the lack of sclerites on the mesonotum points to either Glossosoma or Anagapetus. Although it's difficult to see in a 2D image from the microscope, it's pretty clear in the live 3D view that the pronotum is only excised about 1/3 of its length to accommodate the forecoxa, not 2/3, which points to Glossosoma at Couplet 5 of the Key to Genera of Glossosomatidae Larvae.
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 Drunella coloradensis (Small Western Green Drakes)

This species is very similar to Drunella flavilinea. In areas where their ranges overlap, they can sometimes be found in the same streams. They are similar enough that anglers sometimes refer to either or both species as "Flavs." Allen and Edmunds (1962) say that Drunella coloradensis tends to favor colder water than Drunella flavilinea and that it may emerge as much as a month later.

Where & when

Time of year : July to early October

Preferred waters: Small streams

Altitude: High

According to Fred Arbona, this species requires streams which never exceed 60°F, and this limits them to small streams at high altitude.

In 13 records from GBIF, adults of this species have been collected during September (38%), August (31%), October (8%), June (8%), July (8%), and April (8%).

In 41 records from GBIF, this species has been collected at elevations ranging from 171 to 10984 ft, with an average (median) of 8196 ft.

Species Range


To determine whether a specimen of Drunella belongs to Drunella coloradensis, use the Key to Species of Drunella 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.

Male Spinner

Described in Needham et al (1935) as Ephemerella coloradensis
Body length: 12 mm
Wing length: 12 mm

A species of the fuscata group (now a synonym of Drunella walkeri); closely allied to E. flavilinea (now a synonym of Drunella flavilinea), from which the imago may be distinguished by the dark venation and the straighter second forceps joint, which is not swollen apically.

Imago dark brown, without striking color markings. Thorax dark brown. Venation dark purplish brown. Abdomen rather uniform dark brown; pleural fold pale; margins of segments pale, appearing as light bands when the body is extended. Tails dark brown, deeper brown at the base. Apical margin of forceps base almost straight between the bases of the forceps limbs. Second joint of forceps moderately and regularly curved, not noticeably swollen near the apex; somewhat thinner near the middle than elsewhere.


Described in Needham et al (1935) as Ephemerella coloradensis

Nymph with very low occipital tubercles; tibial ‘thumb’ longer and sharper at the tip than in flavilinea(now a synonym of Drunella flavilinea) ; middle and hind legs relatively more slender; dorsal spines more prominent.

Nymph without frontal horns or frontal shelf. Head roughened; occipital tubercles present, but very low and inconspicuous. No thoracic spines. Anterior margin of the fore femur with teeth or spines. Tibial ‘thumb’ quite long, sharp-pointed, relatively straight. Legs stout, but relatively more slender than in flavilinea; all femora somewhat flattened, the posterior margins with many hairs and a few short spines. Gills borne on segments 3-7. Dorsal spines present on tergites 1-9, somewhat longer on tergites 6-8 than on the basal segments. General color light brown, mottled with paler areas, especially on the legs. Tails each with two dark and two paler bands.

Specimens of the Mayfly Species Drunella coloradensis

1 Male Dun
1 Male Spinner
8 Nymphs

Start a Discussion of Drunella coloradensis


Mayfly Species Drunella coloradensis (Small Western Green Drakes)

Species Range
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