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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Identification: Easy Angler's Key to Common Mayfly Nymphs
Identification: Easy Angler's Key to Common Mayfly Nymphs
This key begins with the useful observation from Al Caucci's excellent Mayfly Guide that two main types of nymphs (swimmers and burrowers) can largely be separated from the other two main types (clingers and crawlers) by the proportions of their major body segments.
| Option 1 | Option 2 |
|---|---|
Abdomen fairly long, so the tips of the wingpads are clearly closer to the front of the head than to the base of the tails in most species 
| Abdomen short by comparison, with the tips of the wingpads being closer to the base of the tails than to the front of the head, or about the same distance from each 
|
| Includes most "burrower" and "swimmer" nymphs | Includes most "clinger" and "crawler" nymphs |
| Remaining families: Ameletidae, Baetidae, Ephemeridae, Isonychiidae, Leptophlebiidae, Polymitarcyidae, Potamanthidae, and Siphlonuridae | Remaining families: Baetiscidae, Caenidae, Ephemerellidae, Heptageniidae, Leptohyphidae, and Leptophlebiidae |
5 Example Specimens
This Isonychia bicolor nymph from the Catskills displays the prominent white stripe sometimes characteristic of its species. This is the first such specimen I've photographed, because members of the same species in the Upper Midwest have a more subdued stripe (and were once thought to be a different species, Isonychia sadleri). The striking coloration on this eastern nymph is more appealing.
I caught this Ameletus nymph with several others of the same kind. This was the most vivid example, but they all had quite a bit of striking and unusual red shading, especially on the last few abdominal segments.
I keyed it out under the microscope using Larvae and adults of Ameletus mayflies (Ephemeroptera: Ameletidae) from Alberta with slightly larger (10 mm), mature specimen with darkened wingpads. Microscope pictures are from that specimen. The characteristics in the key and most of the verification table point pretty clearly to Ameletus cooki, except that the coloration of the antennae more closely resembles Ameletus sparsatus. However, on other characteristics in which these species differ (spines on the dorsal surface of the front femora, which seem very short in this specimen; length of posterolateral spines on segments 8–9; length of spines on posterior edge of tergites 6–9), this is a better match for cooki, and that's probably the correct ID. 
A few of these nymphs turned up in my kick net sample, with this being the only one fully mature and ready to emerge. It keys pretty straightforwardly to Ameletus suffusus, particularly because of the triangular dark patch on the labrum. The abdominal markings are close but don't quite match other drawings, likely just individual variation.
This specimen was collected along with a female which was quite a bit larger, different in color, but otherwise seemed the same morphologically.
These nymphs were highly abundant in my early April kick net samples from the Yakima Canyon, and one of them emerged into a dun, which I photographed immediately. Similar-looking nymphs but with distinctly brighter color patterns were also abundant. I just photographed one. After extensive views under the microscope, it's clear the bright ones are males and the dull ones are females of the same species.
The most likely guess at the species is Baetis tricaudatus, which may be a complex of related species that haven't all been sorted out yet. It isn't a perfect fit to every key characteristic (and I never seem to find a Baetis that matches the expected pronotum color patterns, but that seems to be the closest. The microscope images here were taken with different specimens from the main photos (so I could dissect them while preserving that one intact), but clearly the same species. | 5 Example Specimens
This is an interesting one. Following the keys in Merritt R.W., Cummins, K.W., and Berg, M.B. (2019) and Jacobus et al. (2014), it keys clearly to Ephemerella. Jacobus et al provide a key to species, but some of the characteristics are tricky to interpret without illustrations. If I didn't make any mistakes, this one keys to Ephemerella mucronata, which has not previously been reported any closer to here than Montana and Alberta. The main character seems to fit well: "Abdominal terga with prominent, paired, subparallel, spiculate ridges." Several illustrations or descriptions of this holarctic species from the US and Europe seem to match, including the body length, tarsal claws and denticles, labial palp, and gill shapes. These sources include including Richard Allen's original description of this species in North America under the now-defunct name E. moffatae in Allen RK (1977) and the figures in this description of the species in Italy.
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| Go to Couplet 2 | Go to Couplet 10 |
Adapted from Caucci, Al (2011) and Merritt R.W., Cummins, K.W., and Berg, M.B. (2019)
The current couplet is highlighted with darker colors and a icon, and couplets leading to this point have a icon.
Couplet 1 (You are here)
Leads to Couplet 2:
- Abdomen fairly long, so the tips of the wingpads are clearly closer to the front of the head than to the base of the tails in most species
- Includes most "burrower" and "swimmer" nymphs
Couplet 2
Leads to Couplet 10:
- Abdomen short by comparison, with the tips of the wingpads being closer to the base of the tails than to the front of the head, or about the same distance from each
- Includes most "clinger" and "crawler" nymphs
Couplet 10
Leads to Couplet 3:
- Head has obvious tusks
- Gills feathery, with many fine tendrils
- Body most often yellowish
Couplet 3
Leads to Couplet 5:
- Head usually without tusks
- Gills usually plate-like, although there may be multiple plates or forks
- Body more likely a shade of gray, olive, or brown
Couplet 5
Leads to Potamanthidae:
- Gills held out to the sides of abdomen. I haven't had the opportunity to photograph any of these uncommon mayflies, but a good photo can be found on Jan Hamrsky's Flickr.
- Fairly uncommon
- Length 8–16 mm when mature
Leads to Couplet 4:
- Gills usually held more vertically above abdomen, although they can flex and wave out to the side
- Very common
Couplet 4
Leads to Ephemeridae:
- Tips of tusks curve upward when viewed from the side
- Very large when mature (12–35 mm)
Leads to Polymitarcyidae:
- Tips of tusks curve downward when viewed from the side
- Smaller when mature (9–14 mm)
Leads to Leptophlebiidae:
- Each gill is forked into two sharp, distinct branches
Leads to Couplet 6:
- Gills not forked into two sharp branches, although they sometimes have rounded lobes
Couplet 6
Leads to Couplet 7:
- Gills simple and single (one plate on each side of each abdominal segment)
Couplet 7
Leads to Couplet 8:
- Gills more complex, especially on the first few abdominal segments, with each attachment point having either two plates or a plate and a feathery tuft beneath it
Couplet 8
Leads to Ameletidae:
- Gills with a bold dark line stretching from end to end, roughly parallel and often very close to the edge of the gill
- Tails clearly fringed with fine hairs
Leads to Isonychiidae
(Isonychia):
(Isonychia):
- Forelegs with a thick, basket-like fringe of hairs
- Each gill with a filamentous tuft at its base
- Usually has a pale dorsal stripe, very vivid in some specimens, more diffuse in others
- Usually dark reddish brown color
Leads to Couplet 9:
- No thick basket-like fringe of hairs on forelegs
- Each gill with a pair of plates sharing the same base, often most obvious on the first few abdominal segments
- Wide variety of markings, but not a pale dorsal stripe
- Color more likely grayish-tan or olive
Couplet 9
Leads to Siphlonuridae
(Siphlonurus):
(Siphlonurus):
- Body relatively large, 9–17 mm when mature
- Found more often in rivers, although usually in slower habitats
Leads to Baetidae
(Callibaetis):
(Callibaetis):
- Body smaller, 6–10 mm when mature
- Found more often in lakes, although sometimes abundant in slow-moving river habitats
Leads to Baetiscidae:
- Thorax expanded into a large, rounded shield, reminiscent of a turtle shell, which covers the gills
- Uncommon, but locally abundant on some trout streams
Leads to Couplet 11:
- Thorax not expanded into a turtle-like shell; gills visible on at least some abdominal segments
- Very common
Couplet 11
Leads to Heptageniidae:
- Head distinctly wider than abdomen
- Body and especially head typically flattened, with eyes and antennae on the dorsal surface of the head
Leads to Couplet 12:
- Head about as wide as the abdomen or less
- Body profile closer to cylindrical, not flattened
Couplet 12
Leads to Leptophlebiidae
(Leptophlebia):
(Leptophlebia):
- Broad, leaf-like, double gills on each side of each abdominal segment
Leads to Couplet 13:
- Gills single, or with a plate and feathery tuft beneath it, but not two plates
Couplet 13
Leads to Couplet 14:
- Large, operculate gills attached to abdominal segment 2 extend about half the length of the abdomen
Couplet 14
Leads to Ephemerellidae:
- Most gills are smaller and plate-like with branched tufts beneath However, a few species have enlarged gills attached to abdominal segment 4
Leads to Leptohyphidae:
- Large, operculate gill covers triangular
Start a Discussion of this Couplet
References
- Caucci, Al. 2011. The Mayfly Guide. Scott & Nix, Inc.
- Jacobus, L. M., Wiersema, N.A., and Webb, J.M. 2014. Identification of Far Northern and Western North American Mayfly Larvae (Insecta: Ephemeroptera), North of Mexico; Version 2. Joint Aquatic Science meeting, Portland, OR. Unpublished workshop manual. 1-176.
- Merritt R.W., Cummins, K.W., and Berg, M.B. 2019. An Introduction to the Aquatic Insects of North America (Fifth Edition). Kendall/Hunt Publishing Company.
Families excluded
These families of Ephemeroptera are not included in this key: Metretopodidae, Behningiidae, Neoephemeridae, Acanthametropodidae, Ametropodidae, Arthropleidae, Oligoneuriidae, Pseudironidae, Euthyplociidae, Palingeniidae
Description of this key
This key is helps identify common mayfly nymphs using characteristics that are relatively easy to see and interpret. It omits some rare mayflies and uses characteristics that aren't 100 % reliable, but it will 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 Nymphs.