Yes, appears to be Hex.
Fly fishermen who generally don't fish for bass are often stunned by pictures like this that reveal the magnitude of mayfly hatches on big warm water rivers, and the Susquehanna in particular.
To be sure, I don't regard mayfly hatches on bass rivers the same way I do on trout streams. Bass eat mayflies, but huge bug soup days on rivers often don't produce heavy feeding like one would expect on Penns Creek (there are some glorious days that are exceptions). Nevertheless, pictures like this one are pretty motivating for any FFer! Thanks for posting this.