Identification: Key to Orders of Adult Aquatic Insects, Couplet 6

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.

The setae on the wings make it a big tricky to identify this caddisfly from the "splayed wings" view because they obscure the venation, but several of the side views provide a good look at the important forewing veins. It keys pretty confidently to Ironoquia. Based on species ranges and a photo on BugGuide seemingly of this species and some alternatives, it seems Ironoquia lyrata is by far the most likely option.

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.

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.

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.