Can You Pick the Right Bug?

From the "Even a Blind Hog Finds an Acorn Once in a While" Files:

I had a successful afternoon on the river yesterday, and it only happened after I found the right fly pattern. Honestly, I went from zero to hero in about 20 minutes after making a bug switch. And it wasn't really my first choice... I saw the fly on my hat brim... was too lazy to dig around the box... figured what the heck... and it worked.

I'll set up the situation, and you see if you can figure out what fly pattern it was that turned out to be the haymaker...

I'm in Michigan. Fishing a smaller river. Stumpy. Some gravel. Lots of vegetation at the edges. It's a sunny day. We had rain last night, but the water is still clear. Very clear.

It's that time of year when you get those classic mayfly hatches, like big swarms of gray drakes... but to be honest, the last few nights have been pretty dead. Few bugs, fewer fish.

I pick a fly, and tie it to 5X tippet. It's 2-o'clock in the afternoon. I have no expectations whatsovever... but then it starts going off on that fly.

What fly is it?

Deeter