Quite a meaty morsel indeed. I had been fishing this same watershed just the day before and had a big fish day ( numbers and size) using a Purple Spider and an Iron Blue, both in size 14. Either of these patterns could imitate this natural fly, though I had not noticed the natural while fishing. But I had noticed little black caddis size 16.
And just a few days prior, back at home, I had noticed a swarm of large mayfly spinners over my driveway just at dusk. I now assume those spinners were Leptophlebia cupida.