This large Perlodidae stonefly was a strikingly bright yellow color, more so than any other insect I've seen. I didn't enhance it much. See the discussion threads to follow how we identified this specimen, which was listed incorrectly for several years.
GONZO on Oct 3, 2010October 3rd, 2010, 8:37 am EDT
I believe that this is actually Isogenoides hansoni. When Dr. Chandler and Luke Myers made earlier "Cultus" comments on this specimen, they seem to have missed the prominent submental gills shown in photo #5 (and #7).
In addition to the submental gills (more than 2X as long as wide), the pale area on the 10th tergum, the distinct enclosed pale "M" mark on the head, and (often) the elongated pale spot in the center of the head are a few of the distinctive traits of this species. A complete description and a depiction of the dorsal habitus (figure #88) can be found in Sandberg and Stewart (Holomorphology and Systematics of the Stonefly Genus Isogeniodes, 2005).
The type specimens of I. hansoni were collected from my old homewaters on the Brodheads in 1937 by Preston Jennings (the author of the classic A Book of Trout Flies).
Cultus verticalis is a smaller, pale yellowish nymph, typically with a bolder, simpler dorsal pattern and less intricate markings on the dorsal surface of the head/thorax.