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 one pretty clearly keys to Kogotus, but it also looks fairly different from specimens I caught in the same creek about a month later in the year. With only one species of the genus known in Washington, I'm not sure about the answer to this ID.
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Identification: Key to Families of Stonefly Adults, Couplet 8
Identification: Key to Families of Stonefly Adults, Couplet 8
Error: Tried to access a key that isn't live.
Adapted from Merritt R.W., Cummins, K.W., and Berg, M.B. (2019)
This couplet refers figures (usually designated 'sf') from this source.
You will need the source (make sure to get the same edition!) to see them.
This couplet refers figures (usually designated 'sf') from this source.
You will need the source (make sure to get the same edition!) to see them.
| Option 1 | Option 2 |
|---|---|
Metathoracic sternacostal sutures along posterior margin of basisternum not reaching furcal pits, and obscure arched grooves extend anterolad from furcal pits 
| Metathoracic sternacostal sutures extend laterad from anterior corners of furcal pits to margins of basisternum 
|
Cubitoanal crossvein of forewing usually touching, or very near anal cell
| Cubitoanal crossvein of forewing often absent, or located on the cubitus beyond (distal of) the anal cell by a distance at least equal to the cubitoanal crossvein's own length
|
| Thorax (and sometimes paraprocts) often with branched gill remnants or ragged gill stubs (sf 16.195) | Thorax and paraprocts without branched gill remnants but finger-like (or forked) gills sometimes occur on submentum, thorax, or sides of abdomen (sf 16.198) |
5 Example Specimens
I keyed this specimen to Paragnetina, and range maps would seem to rule out every species of that genus except for the common Paragnetina media.
I found this stonefly on some streamside vegetation. I didn't see any in the air in several hours of fishing.
I caught this female in the vegetation on a trail about 50 feet above the stream. The coloration on its head differs from that of a male I collected recently on a nearby river, and from another female I collected on the same river. But it still keys out confidently to Calineuria, and californica is the only species of that genus in the area.
| 5 Example Specimens
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. 
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. 
Based on photos, this specimen seems to key out (using Merritt & Cummins 4th Edition) to Pictetiella expansa, but some of the characteristics aren't 100 % clear. Frisonia picticeps is another possibility. The range map doesn't show Pictetiella expansa in Washington, but that species is listed elsewhere as occurring near Puget Sound, and the head markings are similar to (albeit not a perfect match for) those of a male from Colorado in the scientific literature.
|
| Perlidae | Perlodidae |
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 Couplet 2:
- First (basal) tarsal segment at least as long as the third (apical) tarsal segment
- Tarsi completely sclerotized on venter
Couplet 2
Leads to Couplet 5:
- First tarsal segment much shorter than third tarsal segment
- Mid and basal tarsal segments with well-developed ventral membraneous pads
Couplet 5
Leads to Taeniopterygidae:
- Second (middle) tarsal segment about as long as first (basal) segment (sf 16.178)
- Gill scar present (sf 16.179) or absent on inner coxal surface
Leads to Couplet 3:
- Second tarsal segment much shorter than first (sf 16.176)
- Gill scar absent from inner coxal surface
Couplet 3
Leads to Capniidae:
- Tails multisegmented (sf 16.180)
- Second anal vein of forewing simple and unforked (sf 16.183)
- Usually 1 or 2 intercubital crossveins
Leads to Couplet 4:
- Tails 1-segmented (sf 16.182)
- Second anal vein of forewing forked (sf 18.183)
- Usually 5 or more intercubital crossveins
Couplet 4
Leads to Nemouridae:
- Apical segment of labial palpus circular and larger than preceding segment (sf 16.184)
- Wings lying relatively flat over the abdomen at rest
- Forewing often with an X-pattern of crossveins at cord (sf 16.183)
- Cervical gills sometimes present (sf 16.185)
Leads to Leuctridae:
- Apical segment of labial palpus similar to preceding segment (sf 16.186)
- Wings rolled around abdomen at rest, giving body a slender, needle-like appearance
- Forewings without an X-pattern of crossveins at cord (sf 16.187)
- Gills absent
Leads to Peltoperlidae:
- Ventroapical tibial spurs small and arranged in one or two rows (sf 16.188)
- Metathoracic sternacostal sutures, if present, arise from posterior corners of furcal pits
- Posterolateral angles of metasterna usually project behind coxae (sf 16.189)
- Two ocelli
Leads to Couplet 6:
- Two slightly enlarged ventroapical spurs occur on each tibia, each surrounded by subapical membraneous area (sf 16.190)
- Sternacostal sutures extend laterad of anterior corners of metathoracic furcal pits, often to margins of basisternum (sf 16.191), or sutures incomplete near furcal pits (sf 16.192)
- Posterolateral angles of metasternum not projecting (sf 16.192)
- Two or three ocelli
Couplet 6
Leads to Pteronarcyidae:
- Gill remnants conspicuous on thoracic sterna between coxae and on first 2 or 3 abdominal sterna (sf 16.193)
- Forewing anal region with two or more rows of crossveins (sf 16.194)
Leads to Couplet 7:
- Thoracic gill remnants, if present, restricted to area behind coxae (sf 16.195) and absent from basal abdominal sterna
- Forewing anal region with, at most, one row of crossveins
Couplet 7
Leads to Chloroperlidae:
- Lateral margins of pronotum not bent downward
- Second anal vein of forewing often forked between anal cell and wing margin (sf 16.196)
- Hind wing anal region usually with less than 5 longitudinal veins
- Body color variable but often green or yellow in life
- Apical maxillary palpal segment often much reduced in size relative to penultimate segment
- Gill remnants absent
Leads to Couplet 8:
- Lateral margins of pronotum usually bent sharply downward, partially covering sides of prothorax (sf 16.197)
- Second anal vein of forewing forked or unforked between anal cell and wing margin
- Hind wing anal region with 5 or more longitudinal veins
- Body color yellow, brown, or black
- Apical maxillary palpal segment not greatly reduced in size
- Gill remnants present or absent
Couplet 8
Couplet 8 (You are here)
Leads to Perlidae:
- Metathoracic sternacostal sutures along posterior margin of basisternum not reaching furcal pits, and obscure arched grooves extend anterolad from furcal pits
- Cubitoanal crossvein of forewing usually touching, or very near anal cell
- Thorax (and sometimes paraprocts) often with branched gill remnants or ragged gill stubs (sf 16.195)
Leads to Perlodidae:
- Metathoracic sternacostal sutures extend laterad from anterior corners of furcal pits to margins of basisternum
- Cubitoanal crossvein of forewing often absent, or located on the cubitus beyond (distal of) the anal cell by a distance at least equal to the cubitoanal crossvein's own length
- Thorax and paraprocts without branched gill remnants but finger-like (or forked) gills sometimes occur on submentum, thorax, or sides of abdomen (sf 16.198)
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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.