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 the first of it's family I've seen, collected from a tiny, fishless stream in the Cascades. The three species of this genus all live in the Northwest and are predators that primarily eat stonefly nymphs Merritt R.W., Cummins, K.W., and Berg, M.B. (2019).
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Stonefly Genus Sweltsa (Sallflies)
Only Sweltsa coloradensis has been specifically mentioned as a common species in my sources.
Genus Range
Specimens of the Stonefly Genus Sweltsa
3 Female Adults
These stoneflies--abundant during midday--seemed at first to be flying around with plain yellow bodies, but on closer inspection turn out to have striking, fiery red abdomens.
In Baumann's Stoneflies of the Rocky Mountains, this specimen keys to Sweltsa fidelis, but the number of species in that key is fairly small and it might not be accurate.
I was not fishing, but happened to be at an unrelated social event on a hill above this tiny creek (which I never even saw) when this stonefly flew by me. I assume it came from there. Some key characteristics are tricky to follow, but process of elimination ultimately led me to Sweltsa borealis. It is reassuringly similar to this specimen posted by Bob Newell years ago. It is also so strikingly similar to this nymph from the same river system that I'm comfortable identifying that nymph from this adult. I was especially pleased with the closeup photo of four mites parasitizing this one.
6 Adults
Another yellow stonefly from Touchet Corral.
5 Nymphs
(Edit: I'm putting a species ID on this one based on the great similarity of its markings to those of an adult I collected in 2023 from a tributary of the same river at around the same time of year.)
This one was tricky to identify. The "thick, depressed dark clothing hairs" on the thoracic sterna inside the coxae, which indicate Sweltsa, are fairly sparse and very difficult to see in my photos, although more apparent under the microscope. However, if I decide those hairs are too sparse to be "thick" and follow the key, I end up pretty clearly at Haploperla. However, Haploperla should have the inner margin of the hind wingpads approximately parallel to the body axis, and they're clearly divergent on this specimen. The pronotal fringe hairs should also be longer than they are in this specimen. So following that whole branch of the key was probably wrong, which leads back to this one being Sweltsa. Unfortunately, nymphs have only been described for a small number of Sweltsa species.
I found just a few of these nymphs alongside a much larger number of bland, typical Sweltsa nymphs.
This one was tricky to identify. The "thick, depressed dark clothing hairs" on the thoracic sterna inside the coxae, which indicate Sweltsa, are fairly sparse and very difficult to see in my photos, although more apparent under the microscope. However, if I decide those hairs are too sparse to be "thick" and follow the key, I end up pretty clearly at Haploperla. However, Haploperla should have the inner margin of the hind wingpads approximately parallel to the body axis, and they're clearly divergent on this specimen. The pronotal fringe hairs should also be longer than they are in this specimen. So following that whole branch of the key was probably wrong, which leads back to this one being Sweltsa. Unfortunately, nymphs have only been described for a small number of Sweltsa species.
I found just a few of these nymphs alongside a much larger number of bland, typical Sweltsa nymphs.
This species was fairly abundant in a February sample of the upper Yakima.
This was by far the most abundant stonefly in my sample on May 14th.
Start a Discussion of Sweltsa
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.
Stonefly Genus Sweltsa (Sallflies)
Taxonomy
Species in Sweltsa
Sweltsa borealisBoreal Sallflies
3
29
Sweltsa coloradensisLittle Yellow Stoneflies
0
0
Sweltsa exquisitaExquisite Sallflies
0
0
Sweltsa fidelisMountain Sallflies
3
8
Sweltsa lateralisCurved Sallflies
0
0
Sweltsa medianaCarolina Sallflies
0
0
Sweltsa naicaNortheastern Sallflies
0
0
Sweltsa occidensAlpine Sallflies
0
0
Sweltsa onkosOntario Sallflies
1
8
Sweltsa oregonensisOregon Sallflies
0
0
Sweltsa pacificaPacific Sallflies
0
0
Sweltsa pocahontasPocahontas Sallflies
0
0
Sweltsa resimaCalifornia Sallflies
0
0
Sweltsa revelstokaCanadian Sallflies
0
0
Sweltsa townesiDardanelle Sallflies
0
0
Sweltsa urticaeNettle Sallflies
0
0
Species in Sweltsa: Sweltsa borealis, Sweltsa coloradensis, Sweltsa exquisita, Sweltsa fidelis, Sweltsa lateralis, Sweltsa mediana, Sweltsa naica, Sweltsa occidens, Sweltsa onkos, Sweltsa oregonensis, Sweltsa pacifica, Sweltsa pocahontas, Sweltsa resima, Sweltsa revelstoka, Sweltsa townesi, Sweltsa urticae
12 species (Sweltsa adamantea, Sweltsa albertensis, Sweltsa californica, Sweltsa continua, Sweltsa cristata, Sweltsa guafini, Sweltsa hondo, Sweltsa lamba, Sweltsa pisteri, Sweltsa tamalpa, Sweltsa umbonata, and Sweltsa voshelli) aren't included.