Identification: Key to Genera of Perlodidae Nymphs, Couplet 10

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.

This nymph keys out to Osobenus yakimae, and a comparison of the markings with a specimen on flyfishingentomology.com provides a good verification. The size is about right, as that species has been described as 12 mm. The markings generally fit the original description of the nymph of that species (Jewett 1955) perfectly.

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 one took an embarrassingly long time to identify, because the key I was following sent me on a wild goose chase when I interpreted the longitudinal dorsal stripe on the abdomen of this specimen as a "longitudinal pigment band." I've updated the key text on this site so that mistake isn't so easy for others to make.

This specimen keys pretty well to Cultus. Key characteristics observed under the microscope but not necessarily apparent on my photo are the lack of submental gills (or any gills at all), the lack of short, stout setae on the occiput or anterolateral prothoracic margins, and the lack of a low knob below the subapical tooth on the lacinia. Species known to be found in Washington are Cultus pilatus and Cultus tostonus. It clearly does not fit the description by Frison (1942) of Diploperla pilata, as Cultus pilatus was first named. I cannot find a detailed description of the nymph of tostonus, but Ricker 1952 describes a defining character of the adults, "Head mostly yellow, the only important dark marking being the bands which join the anterior to the lateral ocelli ; median pronotal stripe, at its middle, about one-fifth of the width of the pronotum." The nymph shows a very dark pattern fitting that description on the head (likely retained into adulthood) and the pronotal stripe is about the right width, too. Given that visual description, the range, and the poor fit to pilatus, I'm calling this Cultus tostonus.

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.