I've done a lot of work with plants, but insects are pretty new and quite challenging for me. I really appreciate your reply and ID comments very much.

Isonychia species are favorites of mine. The pisciform nymphs are the speedsters of the mayfly world, darting about like swift little minnows. They also display an interesting flexibility in their manner of emergence, either emerging at the water's surface in more typical mayfly fashion or crawling out onto streamside rocks to emerge in the manner of most stoneflies. When they crawl out, a heavy emergence will leave tidy little rows of the dark shucks of the nymphs on rocks along the water's edge.

Presently, three Isonychia species seem to be recorded for Nebraska: sicca, rufa, and bicolor. The wings of sicca often have the heavy, dark crossveins seen in your specimen. I. sicca has been found from Minnesota south into Central America.