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 is a striking caddis larva with an interesting color pattern on the head. Here are some characteristics I was able to see under the microscope, but could not easily expose for a picture:
- The prosternal horn is present.
- The mandible is clearly toothed, not formed into a uniform scraper blade.
- The seems to be only 2 major setae on the ventral edge of the hind femur.
- Chloride epithelia seem to be absent from the dorsal side of any abdominal segments.
Based on these characteristics and the ones more easily visible from the pictures, this seems to be Grammotaulius. The key's description of the case is spot-on: "Case cylindrical, made of longitudinally arranged sedge or similar leaves," as is the description of the markings on the head, "Dorsum of head light brownish yellow with numerous discrete, small, dark spots." The spot pattern on the head is a very good match to figure 19.312 of Merritt R.W., Cummins, K.W., and Berg, M.B. (2019). The species ID is based on Grammotaulius betteni being the only species of this genus known in Washington state.
- The prosternal horn is present.
- The mandible is clearly toothed, not formed into a uniform scraper blade.
- The seems to be only 2 major setae on the ventral edge of the hind femur.
- Chloride epithelia seem to be absent from the dorsal side of any abdominal segments.
Based on these characteristics and the ones more easily visible from the pictures, this seems to be Grammotaulius. The key's description of the case is spot-on: "Case cylindrical, made of longitudinally arranged sedge or similar leaves," as is the description of the markings on the head, "Dorsum of head light brownish yellow with numerous discrete, small, dark spots." The spot pattern on the head is a very good match to figure 19.312 of Merritt R.W., Cummins, K.W., and Berg, M.B. (2019). The species ID is based on Grammotaulius betteni being the only species of this genus known in Washington state.
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Caddisfly Genus Onocosmoecus (Great Late-Summer Sedges)
The most common species of this genus is Onocosmoecus unicolor.
Where & when
This genus is nearly as large as Dicosmoecus, but not quite as impressive because its emergence is less concentrated. It can still produce fishable hatches, especially in lakes.In 83 records from GBIF, adults of this genus have mostly been collected during August (35%), July (25%), September (22%), and October (11%).
In 55 records from GBIF, this genus has been collected at elevations ranging from 16 to 10840 ft, with an average (median) of 5092 ft.
Genus Range
Specimens of the Caddisfly Genus Onocosmoecus
2 Adults
1 Male Adult
I first just assumed this was Dicosmoecus based on anglers' conventional wisdom since it's a large orange "October caddis," but Creno set me straight. I should have keyed it out. After another look under the microscope, it lacks an anepisternal wart on the mesopleuron, which rules out Dicosmoecus. The midtibiae have 2 apical spurs and 1 pre-apical spur, and from there the color pattern of the wing points to Onocosmoecus. The location then narrows the species to unicolor.
3 Larvae
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.
See the forum discussion for discussion with a caddis expert (Dave Ruiter / Creno) about the identification of this specimen. I'm a bit perplexed because some gills seem to have more than 4 branches, which would lead in a different direction at Couplet 18 of the Key to Genera of Limnephilidae Larvae, but Dave knew his stuff a lot better than I did. He was convinced this is Onocosmoecus. The current range maps would put this in Onocosmoecus unicolor as the only option, but he believed there are probably multiple not-yet-distinguished species under that umbrella and suggested keeping this one's ID at the genus level for that reason.
Start a Discussion of Onocosmoecus
References
- LaFontaine, Gary. 1981. Caddisflies. The Lyons Press.
- 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.
Caddisfly Genus Onocosmoecus (Great Late-Summer Sedges)
Taxonomy
Species in Onocosmoecus
Onocosmoecus unicolor
4
31
Species in Onocosmoecus: Onocosmoecus unicolor
1 species (Onocosmoecus sequoiae) isn't included.