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.
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Nymphs of this species were fairly common in late-winter kick net samples from the upper Yakima River. Although I could not find a key to species of Zapada nymphs, a revision of the Nemouridae family by Baumann (1975) includes the following helpful sentence: "2 cervical gills on each side of midline, 1 arising inside and 1 outside of lateral cervical sclerites, usually single and elongate, sometimes constricted but with 3 or 4 branches arising beyond gill base in Zapada cinctipes." This specimen clearly has the branches and is within the range of that species.
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Identification: Easy Angler's Key to Common Adult Mayflies, Couplet 9
Identification: Easy Angler's Key to Common Adult Mayflies, Couplet 9
| Option 1 | Option 2 |
|---|---|
Forelegs dark; middle and rear legs pale 
| All legs roughly similar to each other in color 
|
| Remaining families: Ameletidae, Ephemeridae, Heptageniidae, and Siphlonuridae | |
5 Example Specimens
I got several really nice pictures of this spinner. I also collected a female on the same trip.
I collected this female together with a male.
I thought this dun was worth photographing, even though she's in pretty bad shape, because she shows something I haven't seen in Isonychia elsewhere. The bright stripe down the back which is normally present only in the nymph carried over to the dun in this one.
| 5 Example Specimens
Identification of this one was as follows. Body 9 mm, wing 11 mm.
Both Epeorus albertae and Epeorus dulciana should have a conspicuously darkened humeral crossvein in the forewing. This one doesn't. The foretarsal claws are dissimilar (one sharp, one blunt), which also rules out the Epeorus albertae group. The dark macula on the forefemora rules out Epeorus deceptivus, which is also supposed to be a little bit smaller. Both Epeorus grandis and Epeorus permagnus should be much, much larger. Of the species known to be present in Washington, this leaves only Epeorus longimanus, which is exactly the right size. The key to male spinners in Traver (1935) describes distinctive markings that are visible (although more faintly) in this dun: “Black posterior margins of tergites do not extend laterally to pleural fold, but an oblique black line form this margin cuts across poster-lateral triangle to pleural fold.”
This female Hex dun emerged around 11 pm, one of relatively few on the water as I rushed back to the truck to beat an approaching thunderstorm. Her tails in reality were longer than shown in the picture, but they broke off.
For years after I started this website, I was eagerly hoping to find a green drake to add to the collection, but I was never in the right part of the world at the right time. It finally happened on June 1st, 2007.
For details about the tentative identification of this male dun, see the page for associated male spinner, captured from a mating swarm in the same location and at the same time as this dun was emerging.
Although I could not find the preserved specimen to examine under my good new microscope, I'm tentatively calling it one Rhithrogena hageni, based on apparent similarity to this specimen, which I was able to positively ID.
The relative angle of the penes is a bit shallower in this specimen, but I photographed another specimen from the same collecting trip (and I think even the same swarm, although I don't recall for sure) as the other one, and it had the shallower angle seen on this specimen. I'm guessing it's just variation within the species. |
| Isonychiidae Genus Isonychia | Go to Couplet 10 |
The current couplet is highlighted with darker colors and a icon, and couplets leading to this point have a icon.
Leads to Couplet 5:
- Hind wings absent
- Body length 2–7 mm (hook sizes 20 & smaller)
Couplet 5
Leads to Couplet 6:
- Hind wings present
- Body length usually > 7 mm (hook sizes 18 & larger)
Couplet 6
Leads to Baetiscidae
(Baetisca):
(Baetisca):
- Abdomen mostly very thick, then tapering off rapidly in the last 4 segments
- Hind wing rounded, almost circular
- Wings of dun blotchy, wings of spinner clear
- Body length 7–10 mm (hook sizes 14–18)
- Fairly uncommon
Leads to Couplet 8:
- Abdomen thinner in proportion to the rest of the body; more "normal" mayfly shape
- Hind wing more oblong and often less rounded, more angular
- Wings variable
- Length variable
- Very common
Couplet 8
Leads to Leptohyphidae
(Tricorythodes):
(Tricorythodes):
- Abdomen black, dark brown, or olive green
- Wings without a black stripe on the leading edge, although thick veins there may be darker than the rest of the wing
- Male tails around twice the length of the body
- Very common on many trout streams
Leads to Ephemeridae
(Ephemera):
(Ephemera):
- Wings usually with irregular, blotchy marks
- Very large bodies typically at least 15 mm long (hook size 8 or larger)
- Body with a very elongated abdomen, almost serpentine; the "drake" shape characteristic of their long-bodied burrowing nymphs
- Hind wing large, typically one third to one half the length of forewing
Leads to Couplet 7:
- Wings of dun usually shades of uniform gray; wings of spinner usually clear
- Medium-large to small bodies typically less than 15 mm long (hook size 10 or smaller), although the largest species can just reach hook size 8
- Body with more "average" mayfly proportions, not exceptionally elongated
- Hind wing size variable, but typically about one quarter to one third the length of forewing
Couplet 7
Leads to Ephemerellidae:
- Hind wings with a distinct costal angulation, i.e. a small, rounded bump in the front edge
Leads to Leptophlebiidae:
- Hind wings without costal angulation, except in some species rarely found on trout streams
- Continue to Couplet 2 of the Key to Genera of Male Leptophlebiidae Spinners to easily narrow it down one step further.
Leads to Polymitarcyidae
(Ephoron):
(Ephoron):
- All legs of female, and all except front legs of male, feeble and non-functional
Leads to Couplet 9:
- Legs normal and fully functional
Couplet 9
Couplet 9 (You are here)
Leads to Isonychiidae
(Isonychia):
(Isonychia):
- Forelegs dark; middle and rear legs pale
Leads to Couplet 10:
- All legs roughly similar to each other in color
Couplet 10
Leads to Couplet 11:
- Intercalary veins between vein CuA and CuP in the forewing consist of a series of small veinlets, often forking or curved, that connect vein CuA directly to the hind margin of the wing
Couplet 11
Leads to Couplet 12:
- Intercalary veins in this region variable, but not as at left
Couplet 12
Leads to Siphlonuridae
(Siphlonurus):
(Siphlonurus):
- Eyes of male with a pale, horizontal band
- Hind wings with a rounded costal angulation
Leads to Ameletidae
(Ameletus):
(Ameletus):
- Eyes of male without a pale, horizontal band
- Hind wings with a pointed costal angulation
Leads to Ephemeridae
(Hexagenia):
(Hexagenia):
- The largest North American mayflies, with typical body lengths greater than 17 mm
- Body with a very elongated abdomen, almost serpentine; the "drake" shape characteristic of their long-bodied burrowing nymphs
- Hind wing large, typically almost half the length of forewing
Leads to Heptageniidae:
- Body lengths very from small to medium-large, but almost always less than 17 mm
- Body proportions may have a very "average" mayfly shape Alternatively, in many cases, the head, legs, or body might appear somewhat flattened and splayed-out, reflecting the flattened profile of their clinger-type nymphs
- Hind wing size variable, typically around one quarter to one third the length of forewing
Start a Discussion of this Couplet
References
- Baumann, Richard W. 1975. Revision of the Stonefly Family Nemouridae (plecoptera) : a Study Of The World Fauna At The Generic Level. Smithsonian Contributions to Zoology undef(211): 1-74.
Families excluded
These families of Ephemeroptera are not included in this key: Metretopodidae, Behningiidae, Neoephemeridae, Potamanthidae, Acanthametropodidae, Ametropodidae, Arthropleidae, Oligoneuriidae, Pseudironidae, Euthyplociidae, Palingeniidae
Description of this key
This key is helps identify common adult mayflies using characteristics that are relatively easy to see and interpret. It omits many rare mayflies and uses characteristics that aren't all 100 % reliable, but it should put anglers close to the right answer most of the time. However, when accuracy is critical, you should use the full Key to Families of Mayfly Duns and Spinners.