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 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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Stonefly Genus Isoperla (Stripetails and Yellow Stones)
This is a very common trout stream stonefly genus. It is also huge, with sixty species scattered over the entire continent. Though anglers usually call them Little Yellow Stones or Yellow Sallies, the scientific common name for this genus is Stripetail. The latter is probably better to use as not all of them are necessarily little or yellow.
Genus Range
Specimens of the Stonefly Genus Isoperla
5 Adults
Several stoneflies of this species were flying around a small stream last night. I tied on a brown drake imitation of about the right size/color to approximate these stones, and was rewarded with a hard-fighting 17-inch brown trout.
A friend brought me this stonefly to photograph. He found it floating on the surface of a trout stream with its wings in a crippled position.
An adult stonefly carrying a load of red mites.
3 Male Adults
Family-level ID following Merritt, Cummins, & Berg 5th Edition:
1. Thoracic gill remnants absent
2. Arms of mesosternal Y-ridge approach posterior end of furcal pits
3. Submental gill remnants short, obscure, or absent
4. Male. Note that sternum 9 (the long, dark, last segment) appears to be the 8th if you count from the front in the ventral view, because the first segment is not easily visible ventrally.
5. Male abdominal tergum 10 not divided posteriorly; Paraprocts modified as hooks.
6. Male abdominal tergum 10 simple without notches or other prominent processes. This one was tricky, because at first glance it looks like the hooks arise from tergum 10, but in fact they're coming from the paraprocts underneath. There's a medial groove on tergum 10 that could be seen as a "notch" too, but it seems by "notch" the key is referring to a notch in the margin.
7. Abdominal sternum 8 with a conspicuous lobe and sternum 7 without such a lobe.
This leads to Isoperla.
Among Isoperla species known in Washington, most species are ruled out by different shapes of the vesicle (the rounded posterior bump on sternum 8). Isoperla pinta is ruled out by the lack of a region of stout spinules on tergum 9. The remaining options (without knowing how to properly dissect and image the aedeagus) are Isoperla fulva, Isoperla marmorata, Isoperla tilasqua, and Isoperla gravitans, the latter of which is too large. The other three are all at least slim possibilities, but several described characteristics seem to best match Isoperla fulva, which is also mentioned as the most common western Isoperla.
1. Thoracic gill remnants absent
2. Arms of mesosternal Y-ridge approach posterior end of furcal pits
3. Submental gill remnants short, obscure, or absent
4. Male. Note that sternum 9 (the long, dark, last segment) appears to be the 8th if you count from the front in the ventral view, because the first segment is not easily visible ventrally.
5. Male abdominal tergum 10 not divided posteriorly; Paraprocts modified as hooks.
6. Male abdominal tergum 10 simple without notches or other prominent processes. This one was tricky, because at first glance it looks like the hooks arise from tergum 10, but in fact they're coming from the paraprocts underneath. There's a medial groove on tergum 10 that could be seen as a "notch" too, but it seems by "notch" the key is referring to a notch in the margin.
7. Abdominal sternum 8 with a conspicuous lobe and sternum 7 without such a lobe.
This leads to Isoperla.
Among Isoperla species known in Washington, most species are ruled out by different shapes of the vesicle (the rounded posterior bump on sternum 8). Isoperla pinta is ruled out by the lack of a region of stout spinules on tergum 9. The remaining options (without knowing how to properly dissect and image the aedeagus) are Isoperla fulva, Isoperla marmorata, Isoperla tilasqua, and Isoperla gravitans, the latter of which is too large. The other three are all at least slim possibilities, but several described characteristics seem to best match Isoperla fulva, which is also mentioned as the most common western Isoperla.
These Isoperla are loaded with red parasitic mites.
1 Female Adult
The family ID on this one was a little bit tricky. Just going by the size, shape, and color, it looks like Chloroperlidae. However, the second anal vein of the forewing is does not appear to be forked, and the apical maxillary palpal segment is close to the length of the penultimate segment, both of which rule out that family. The position of the cubitoanal crossvein relative to the anal cell in the forewing -- touching it in this case -- indicates Perlidae (and it really doesn't have the "look" of Perlidae at all), but other characteristics, such as the metathorastic sternacostal sutures and lack of gill remnants, point to Perlodidae. That's the right answer. Moving on to Perlodidae, the key characteristics in Merritt & Cummins lead straightforwarly to Isoperla, and the species key in Jewett 1959 (The Stoneflies of the Pacific Northwest) leads to Isoperla fusca.
There is one caveat: That source does suggest a May-July emergence, whereas this one was collected in mid-September.
There is one caveat: That source does suggest a May-July emergence, whereas this one was collected in mid-September.
8 Nymphs
This specimen represents a common find in a late-April sample from the far upper Yakima River. It seems to be the same species as another one I collected previously. Of the species keyed in Szczytko & Stewart 1979, it probably matches Isoperla fusca closest, but there's a good chance it's a species that wasn't in the key. The leg segments have a fringe of fine hairs which is supposed to be absent in Isoperla fusca, and the four dark stripes of the mesonotum and metanotum don't continue as 4 separate stripes on the pronotum as they should in the description of fusca. It's possible fusca is more variable than previously described, or this is a different species not included in that key. It's also worth noting there's definitely no fringe of fine setae on any part of the cerci, just the whorls of little stout ones around segment bases.
This specimen keys to Isoperla fusca using the key in Szczytko & Stewart 1979, but some species of Isoperla weren't represented there. It's probably the same species as this one collected a year later. Despite taking many good pictures of both, I'm not sure of the species ID.
Size - (excluding tails) - 10 mm
Status at time of photo - preserved but no discernible color change as specimen was captured only a few days earlier.
Key characters - unique pattern on dorsal head and thorax; dark medial terga stripe between two paler stripes
This species is very common in west slope north Sierra watersheds. Richard W. Baumann & Boris C. Kondratieff did a study on the same section and at the same time of year (APRIL 25-29, 2010) where this specimen was taken. No other species of Isoperla were reported from this location and quinquepunctata was very abundant. Their hatches can be very heavy some Springs.
Entoman
Status at time of photo - preserved but no discernible color change as specimen was captured only a few days earlier.
Key characters - unique pattern on dorsal head and thorax; dark medial terga stripe between two paler stripes
This species is very common in west slope north Sierra watersheds. Richard W. Baumann & Boris C. Kondratieff did a study on the same section and at the same time of year (APRIL 25-29, 2010) where this specimen was taken. No other species of Isoperla were reported from this location and quinquepunctata was very abundant. Their hatches can be very heavy some Springs.
Entoman
This is probably Isoperla petersoni, a very common Perlodid stonely in Alaska streams.
Start a Discussion of Isoperla
References
- 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.
- Schwiebert, Ernest G. 1955. Matching the Hatch. MacMillan Publishing Company.
Stonefly Genus Isoperla (Stripetails and Yellow Stones)
Taxonomy
Species in Isoperla
Isoperla aculaFresno Stripetail
0
0
Isoperla aduncaArroyo Stripetail
0
0
Isoperla baumanniCalifornia Stripetail
0
0
Isoperla bifurcataForked Stripetail
0
0
Isoperla bilineataTwo-lined Stripetails
0
0
Isoperla burksiBanded Stripetail
0
0
Isoperla denningiAngeles Stripetail
0
0
Isoperla dicalaSable Stripetails
0
0
Isoperla fulvaWestern Stripetails
5
28
Isoperla fuscaWaterton Stripetails
3
37
Isoperla gibbsaeQuebec Stripetail
0
0
Isoperla holochloraPale Stripetail
0
0
Isoperla marlyniaMidwestern Stripetails
0
0
Isoperla marmorataRed Stripetail
0
0
Isoperla miwokMiwok Stripetail
0
0
Isoperla montanaMontane Stripetail
0
0
Isoperla mormonaMormon Stripetail
0
0
Isoperla pintaCheckered Stripetails
0
0
Isoperla quinquepunctataLittle Yellow Stoneflies
2
5
Isoperla rainieraRainier Stripetail
0
0
Isoperla roguensisRogue Stripetail
0
0
Isoperla signataTransverse Stripetails
0
0
Isoperla similisBlack Stripetail
0
0
Isoperla sobriaColorado Stripetails
0
0
Isoperla sordidaNotched Stripetails
0
0
Isoperla tilasquaOregon Stripetail
0
0
Species in Isoperla: Isoperla acula, Isoperla adunca, Isoperla baumanni, Isoperla bifurcata, Isoperla bilineata, Isoperla burksi, Isoperla denningi, Isoperla dicala, Isoperla fulva, Isoperla fusca, Isoperla gibbsae, Isoperla holochlora, Isoperla marlynia, Isoperla marmorata, Isoperla miwok, Isoperla montana, Isoperla mormona, Isoperla pinta, Isoperla quinquepunctata, Isoperla rainiera, Isoperla roguensis, Isoperla signata, Isoperla similis, Isoperla sobria, Isoperla sordida, Isoperla tilasqua
34 species (Isoperla bellona, Isoperla conspicua, Isoperla cotta, Isoperla coushatta, Isoperla davisi, Isoperla decepta, Isoperla decolorata, Isoperla distincta, Isoperla emarginata, Isoperla extensa, Isoperla francesca, Isoperla frisoni, Isoperla grammatica, Isoperla gravitans, Isoperla irregularis, Isoperla jewetti, Isoperla katmaiensis, Isoperla lata, Isoperla longiseta, Isoperla major, Isoperla maxana, Isoperla mohri, Isoperla namata, Isoperla nana, Isoperla orata, Isoperla ouachita, Isoperla petersoni, Isoperla phalerata, Isoperla punctata, Isoperla richardsoni, Isoperla sagittata, Isoperla slossonae, Isoperla szczytkoi, and Isoperla transmarina) aren't included.