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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 spinifera (Western Slate Olive Duns)

This taxon prefers cold water and does not flourish where water temperatures exceed 60 degrees. As with the slightly larger Drunella coloradensis, this species prefers cold tailwaters, high elevation headwaters and spring creeks.

Where & when

Time of year : August and September

This species is most prevalent near the Pacific Coast, but scattered local populations occur throughout the northern states of the West.

In 1 records from GBIF, adults of this species have been collected during November (100%).

In 2 records from GBIF, this species has been collected at elevations of 1982 and 3300 ft.

Species Range

Spinner behavior

Time of day: Dusk

These spinner flights are too sparse to be important.

Identification

To determine whether a specimen of Drunella belongs to Drunella spinifera, 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 Nymph

Described in McDunnough, J (1934) as Ephemerella spinifera
Body length: 11 mm

Color light brown, but this is probably due to fading in the alcohol and fresh specimens may be considerably darker and show an abdominal pattern. Head with a pair of long, erect, pointed tubercles on vertex, about as long as distance separating them. Pronotum with a twin pair of tubercles on each side subdorsally, situated on posterior half of segment, the anterior one being twice as high as the posterior one, both pointed; a lateral pointed tubercle at same level as anterior subdorsal one and somewhat smaller than this.

Mesonotum with a small pair of rounded wart-like tubercles, situated subdorsally on anterior margin; laterad and slightly posterior to these on each side is a longer, sharply pointed tubercle, directed slightly backwards; in central area is a pair of long, pointed subdorsal tubercles, twice as large as the preceding pair, and also slightly slanting backwards, and there is a single median rounded wart-like prominence on the posterior portion, equivalent to the scutellum of the adult; in the latero-anterior angle is a small rounded wart, separated by a slight excision from a still smaller wart, placed directly behind it; somewhat dorsad of these is a slight raised ridge.

On abdomen dorsally segments 2-7 bear each a pair of sharply pointed, spine-like tubercles on the posterior margin; these are all practically equal in length and directed slightly backward; the two on segment II are closer together than those on the following segments and on segments 1V-VII the distance between the tubercles is slightly greater than their height. On segments VIII and IX the spine-like tubercles are enormously increased in size and fully four times the length of the preceding pairs; they are pointed and covered with long, scattered bristles, the pair on VIII faintly divergent apically, those on IX slightly smaller than on VIII, subparallel and rather more decumbent. Tails light brown in basal fourth, broadly ringed with darker brown in outer three-quarters. Lateral spine-like prolongations of abdominal segments III-VII increasing in size from front to rear; lateral margin of VII slightly convex, of VIII and IX distinctly sinuate, the prolongations on these last two segments being subequal and somewhat longer than on VII. Ventrally on abdomen are sub-lateral rows of dark brown dashes, one dash to each segment. Legs pale brown, the femora of two posterior pairs showing traces of two dark transverse bands on outer sides; hind tibia equal in length to femur and trochanter combined.

Male Spinner

Described in Needham et al (1935) as Ephemerella autumnalis
Body length: 11 mm
Wing length: 15 mm

A species of the fuscata (now a synonym of Drunella walkeri) group, allied to E. spinifera (now a synonym of Drunella spinifera).

Eyes deep reddish brown. Head black; pits at bases of antennae "and a small area posterior to mid-ocellus pale yellowish. Pronotum black with yellowish shades laterally” (McD.) at bases of fore legs. Mesonotum black; pale yellowish brown markings as indicated; a “lateral streak extending forward from base of wing, the lateral edge of the anterior projection, the lateral posterior edges reaching caudad to beginning of scutellum and relieving in this region a black dorsal band as broad as and extending over scutellum” (McD.). Metathorax black; slight yellowish shading on scutellum, intersegmental areas of pleura and around bases of legs. Fore femur and tibiae light amber, “heavily suffused with blackish, the pale color only showing at bases of femora and at joint” (McD.); tibia subequal to femur; tarsus light smoky amber. Middle and hind femora and tibia light amber, the femora suffused with blackish in distal third; tibia as long as femur and trochanter together; tarsi and claws dull smoky. Wings hyaline, tinged faintly with smoky in stigmatic area. Veins and cross veins rather fine, dark; stigmatic cross veins paler.

Abdominal tergites 1 and 2 almost wholly black; 3-7 blackish, anterior margins narrowly pale smoky, these are broader at median area of tergite 7. Traces “of a geminate dark median line filled with pale color; on segments 5-7 there is a tendency for the black area to break up into three patches, due to paler submedian prongs projecting from the posterior pale area into and almost separating the dark area; Iaterally there are faintly pale comma-shaped marks in the dark areas” (McD.). Tergites 8 and 9 all black except a narrow pale posterior margin; 10 pale, "narrowly dark on anterior border with median and lateral prongs projecting backward into the pale area” (McD.). Sternites 2-7 with a narrow pale anterior margin and wider pale posterior margin, blackish between these pale bands; two pale submedian lunate marks on anterior margin, two small dots near center of segment, and a larger lateral pale dot or dash, on dark portion of each sternite. Sternites 8 and 9 black; posterior margin of 9 pale. Forceps blackish; yellowish brown apically; base pale yellowish brown. Tails black.

Nymph

Described in Needham et al (1935) as Ephemerella autumnalis

Nymph quite similar to E. spinifera (now a synonym of Drunella spinifera), but differing in these points: twin spines on pronotum subequal; postero-lateral spines of segments 8 and 9 shorter, most evident of 9; long dorsal abdominal spines on segment 8 “more strongly bent backward and downward” (McD.). The species matures fully a month later than spinifera.

Specimens of the Mayfly Species Drunella spinifera

1 Female Dun
1 Male Spinner
2 Nymphs

Start a Discussion of Drunella spinifera

References

Mayfly Species Drunella spinifera (Western Slate Olive Duns)

Taxonomy
Species Range
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