Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
on the Little Juniata last week we happened upon a thick hatch of large black caddis with mottled gray wings. they sure weren't the grannoms we're used to. they were about a size 10, and by the time we'd found something at the bottom of the fly boxes to try, the fish were so full they wouldn't come up any more.
Entoman on Apr 5, 2012April 5th, 2012, 11:44 am EDT
Hopefully, somebody with more familiarity with that stream will check in. The only thing that I can say is that there are five species of Brachycentrus (Grannom) reported in PA, so it doesn't surprise me that you may be seeing a species you haven't noticed before. You've also got a lot of rhyacophilids as well that may be possibilities. As to the black body with mottled gray wings, I'm not aware of any caddis with that combination in a size 10, though several species that have been around awhile as adults can have fairly dark bodies. Too bad you couldn't get a photo.:)
"It's not that I find fishing so important, it's just that I find all other endeavors of Man equally unimportant... And not nearly as much fun!" Robert Traver, Anatomy of a Fisherman