Western March Brown
Rhithrogena morrisoni
Key Identification Characters - nymph 2
- Body shape quite flat; head as wide or slightly wider than the thorax with eyes set on top of head.
- Three tails.
- Gills large and overlapping. First and last pair overlap underneath abdomen forming a sucker like disc.
- Body length of mature nymphs (excluding tails) 5-12mm.
- Body color olive-brown to dark reddish-brown.
Similar nymphs: 2
Rhithrogena morrisoni is classified in the clinger group of mayflies, all of which belong to the family Heptageniidae. All species in this family have a similar flattened body shape with a head as wide or wider than the thorax. The large overlapping gills and three tails separate Rhithrogena from other Heptageniidae. Several species of the genus Epeorus also have large overlapping gills, but have only two tails instead of three. R. morrisoni can be distinguished from other Rhithrogena species (10 known in the West) by its early spring emergence period.
Key Identification Characters - adult 2
- Two tails.
- Wide flat head reflecting the nymph’s head shape.
- Front and hind wings well developed with mottled brown color (dun) or clear (spinner).
- Body color tan to pale brown below and brown to reddish brown above.
- Body length 6-15mm.
Similar Winged Mayflies: 2
The key features of Heptageniidae adults are their two tails and flattened head. The mottled brown wings of Rhithrogena morrisoni duns and its early spring emergence period separate it from other Heptageniidae species. Collecting nymphs or nymphal shucks will also help identify the duns present.
Habitat & Distribution: 2
Rhithrogena morrisoni prefers riffles and glides with baseball to bowling ball sized substrate in moderate to large rivers throughout the West. As streams become smaller, steeper, faster, and have larger substrate other species of Rhithrogena typically become more dominant.
In Oregon Rhithrogena morrisoni can be found across the state, but is particularly common and abundant in the moderate to large Willamette Valley streams like the Santiam, Mollala, McKenzie, and mainstream Willamette. Streams in central and eastern Oregon like the Deschutes also have good populations.
Life Cycle - Emergence 2
Rhithrogena morrisoni is the first larger mayfly to emerge in the late winter or early spring following or overlapping with the small blue-winged olive (Baetis sp) mayflies. Emergence begins when water temperatures rise to about 47 or 48 degrees. When water temp hits 50 emergence will be in full swing. The number emerging tends to vary widely from one day to the next depending on weather conditions. Mild stable weather with overcast skies generally produces large hatches. In the Willamette Valley spring storms and high streamflows often occur right when R. morrisoni emerges, interfering with hatches and fishing.
R. morrisoni is univoltine (one generation per year), and emergence can spread out over over a three to six week period.
Oregon Emergence Period: late March to mid May.
Metolius: ?
McKenzie: Late March to mid May with peak generally early to mid April.
Necanicum/Nehalem: ? Probably late March to May
Sources and Credits
- (c) Jerry Schoen, all rights reserved, uploaded by Jerry Schoen
- (c) Jerry Schoen, some rights reserved (CC BY-SA)
