This is the same species that makes the midrib galls I described below.

I'm pretty sure this is also a midge gall, but a different species. I've also been seeing midge galls in the middle of jewelweed leaves (on the underside of the midrib), which may or may not be yet another species. I'll let you know (if no one else does first) when my copy of this book shows up. I had it checked out from a library before, and it's a great resource if you can find a copy. I had to order a used copy from the U.K.