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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This specimen keys pretty easily to Onocosmoecus, and it closely resembles a specimen from Alaska which caddis expert Dave Ruiter recognized as this genus. As with that specimen, the only species in the genus documented in this area is Onocosmoecus unicolor, but Dave suggested for that specimen that there might be multiple not-yet-distinguished species under the unicolor umbrella and it would be best to stick with the genus-level ID. I'm doing the same for this one.
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Identification: Key to Orders of Aquatic Insect Nymphs, Couplet 1
Identification: Key to Orders of Aquatic Insect Nymphs, Couplet 1
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Adapted from Merritt R.W., Cummins, K.W., and Berg, M.B. (2019)
| Option 1 | Option 2 |
|---|---|
Chewing mouthparts, with mandibles distinct 
| Sucking mouthparts, united in a jointed beak with mandibles concealed 
|
| Remaining orders: Ephemeroptera, Odonata-Anisoptera, Odonata-Zygoptera, Orthoptera, and Plecoptera | |
5 Example Specimens
This striking golden stonefly is the first of its species I've had the chance to photograph.
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.
| |
| Go to Couplet 2 | Hemiptera |
Adapted from 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.
Leads to Orthoptera:
- Hind legs suitable for jumping, with hind femora greatly enlarged
- Abdomen with short tails
- Found in moist places and only temporarily in water
Leads to Couplet 3:
- Hind legs suited for crawling, with hind femora similar in size to front and middle femora, not greatly enlarged
- Abdomen with or without conspicuous tails
- Usually submerged and truly aquatic
Couplet 3
Leads to Couplet 5:
- Labium (lower lip) mask-like, extendable into a scoop-like structure longer than head
Couplet 5
Leads to Couplet 4:
- Labium normal, smaller than head, not large and mask-like
Couplet 4
Leads to Ephemeroptera:
- Tarsi almost always with one claw
- Usually 3 tails, but sometimes 2
- Plate-like, filamentous, or feathery gills located on sides of abdomen, often present on most middle abdominal segments
Leads to Plecoptera:
- Tarsi with 2 claws
- Always 2 tails
- Finger-like or feathery gills present, but sometimes inconspicuous at base of mouthparts, head, legs, first few abdominal segments, or last abdominal segment
Leads to Odonata-Zygoptera:
- Body slender, with head wider than thorax and abdomen
- 3 long, caudal tracheal gills at the tip of the abdomen resemble paddle-like tails
Leads to Odonata-Anisoptera:
- Body stout, with head usually narrower than thorax and abdomen
- 5 short, stiff, pointed appendages at the tip of the abdomen
Start a Discussion of this Couplet
References
- 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.
Orders excluded
These orders of Insecta are not included in this key: Trichoptera, Diptera, Coleoptera, Megaloptera, Lepidoptera, Hymenoptera, Neuroptera, Psocodea