Hex
Hexagenia limbata
Key Identification Characters - Nymph 3
- Large mandibular tusks projecting from front of head; front legs large and designed for digging.
- Three tails fringed with fine hairs.
- Gills on segment one small and forked; those on segments 2-7 large & triangular in shape, fringed with hairs, and overlap on top of abdomen.
- Large. Body length of mature nymphs (excluding tails) 15-37mm.
- Body yellowish-brown with dark markings on top of abdomen.
Similar nymphs: 3
The large size and distinct shape of Hexagenia limbata make it difficult to confuse with nearly any other mayfly nymph. Hexagenia nymphs are burrowers and only two other species of burrowers are found in the West: Ephemera simulans (Brown Drake) and Ephoron album (White Drake). Both of these species, while large by mayfly standards, are distinctly smaller than Hexagenia limbata and live in different habitat conditions.
Key Identification Characters - Adult 3
- Two tails.
- Front and hind wings well developed, pale yellow in color (dun) or clear with a faint yellow tinge (spinner).
- Body color pale yellowish-brown to nearly bright yellow with distinct dark bands on top of abdomen and thorax.
- Body length (excluding tails) 16-37mm.
Similar Winged Mayflies: 3
Hexagenia limbata is the largest mayfly found in the West, and size alone make it distinct. Add the yellow color and large yellowish wings and identification becomes rather simple; there is just no other mayfly in the West of comparable size and color. The Brown Drake (Ephemera simulans) comes closest in size and color to Hexagenia, but adult Brown Drakes have three tails instead of two, so if in doubt take a close look at the number of tails.
Habitat & Distribution 3
Hexagenia limbata nymphs need a substrate of soft sandy mud - not too hard, not too soft, just right - to make their u-shaped tunnels into the bottom. Whether this is in a lake or stream makes little difference to them and good populations are found in both. Because of the specific habitat needs Hexagenia have a spotty distribution throughout the West, but where conditions are right they can be extremely abundant.
In Oregon Hexagenia limbata occurs across the state in both lakes and streams, but again only where the proper habitat conditions prevail. Streams like the upper Williamson in south-central Oregon and coastal lakes such as Tenmile north of Coos Bay, are well known for their large hatches of Hexagenia.
Life Cycle - Emergence 3
The key to finding Hexagenia emerging is first finding a stream or lake with the proper habitat and thus a good population of nymphs, and then waiting until after dark. Nymphs swim up from the bottom and duns emerge in the surface, but only under very low light conditions. On very dark cloudy days this can occur as early as late afternoon or early evening, but on bright sunny days the emergence will not start until twilight or even complete darkness. Even in such low light, seeing these giant duns pop up on the surface and fly off is an impressive sight.
H. limbata is semivoltine (two to three years per generation), and emergence can spread out over over a three to six week period.
Oregon Emergence Period: mid June to late August.
Metolius: Not present
McKenzie: Not present
Necanicum/Nehalem: Not present
Sources and Credits
- (c) Jenn Forman Orth, some rights reserved (CC BY-NC-SA), http://www.flickr.com/photos/49503155549@N01/2640340536
- (c) Jerry Schoen, all rights reserved, uploaded by Jerry Schoen
- (c) Jerry Schoen, some rights reserved (CC BY-SA)
