Pale Morning Dun
Ephemerella excrucians
Key Identification Characters - nymph: 2
- Body shape somewhat rectangular with small oval shaped gills resting on top of abdominal segments 3-7.
- Three tails with dense fringe of fine hairs laterally the last 1/2 to 2/3 of their length.
- Body length of mature nymphs (excluding tails) 6-12mm.
- Body color highly variable ranging from reddish-brown to almost black. Legs banded.
Similar nymphs: 2
The family Ephemerellidae contains several similar looking genera and species. Most easily confused with PMD nymphs is the species Serratella tibialis, often called the Small Western Dark Hendricksons. Serratella nymphs look nearly identical to PMD nymphs, but lack the fringe of fine hairs on the lateral margins of the tails.
Key Identification Characters - Adult 2
- Three tails.
- Hind wings with distinct rounded costal angulation (small hump on leading edge).
- Wing color pale yellowish-gray to light gray (duns) or clear (spinners).
- Body color variable ranging from bright yellow to olive-yellow to reddish-brown.
- Body length 6-12mm.
Similar winged mayflies: 2
Like the nymphs the most likely species to be confused with PMD adults is Serratella tibialis adults. The most obvious difference is the darker olive-brown body of Serratella duns.
Habitat & Distribution 2
Ephemerella excrucians is adapted to a wide range of stream types with excellent populations found in small streams to large rivers and spring creeks to tailwaters, from the Rocky Mountains to the Pacific Coast. Nymphs live in shallow, moderate to fast riffles with gravel bottoms as well as slow moving reaches where aquatic plants are abundant.
As elsewhere in the West, PMDs are found in streams throughout Oregon. Before recent taxonomic revisions there were two species of western PMDs: Ephemerella inermis and E. infrequens. These species are now both considered to be Ephemerella excrucians. E. excrucians is one of the most widespread and abundant mayflies living in Oregon’s streams.
Life Cycle - Emergence 2
Dun emergence occurs when mature nymphs swim up to the surface, or somewhat below the surface, in which case the newly emerged duns rise up through the water and break through the surface to dry their wings . Dun emergence can occur from mid morning to late afternoon depending on weather conditions. Large numbers on the surface is common. After a day or two duns molt to the spinner stage. Spinners mate in the air, and then lay their eggs. Good spinner falls can occur from early to mid morning or late afternoon to evening depending on weather conditions.
PMDs are univoltine (one generation per year), and emergence can spread out over over a four to six week period.
Oregon Emergence Period: late May - early July.
Metolius: June - July
McKenzie: late May - end of June
Necanicum/Nehalem: late May - end of June?
Sources and Credits
- (c) Jerry Schoen, all rights reserved, uploaded by Jerry Schoen
- (c) Jerry Schoen, some rights reserved (CC BY-SA)
