The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
These two exuviae are my sad little consolation prize after collecting a couple of really cool, large, mature mayfly nymphs I didn't recognize from a tiny, probably fishless, spring-fed tributary of a slightly less tiny trout stream. I saw the nymphs in the kicknet samples and carefully transferred each one to my holding cooler before going to collect more samples. When I got home, I carefully went through the whole sample and couldn't find either one of them. Getting worried, I did it again and even more carefully. This time, I found both exuviae. By process of elimination, pretty much the only possibility is that both of them hatched out of my cooler and flew away during the 10 minutes or so that I had the lid off to collect more samples.
I was really excited about them for the whole three-hour drive home and seriously bummed that they disappeared. In the field I thought they might be one of the families of swimmer nymphs I don't have yet, like Ametropodidae, but keying the exuviae takes me to Ameletus, so at least I didn't miss some great rarity. Still, they were much larger and darker than any other Ameletus I've collected and probably a species I don't have yet.
Edit: I returned four years later and caught a specimen of Ameletus velox in this spot (actually several, just photographed one), which I think solves the mystery of this one's ID.