I was shooting the little tiny fly on this dandelion, right up on top of it, when suddenly this giant hoverfly landed in the middle of my picture. I only got a couple of snaps before it flew off again as abruptly as it landed.

I've never seen one like this before, and my Audobon guide to insects has failed me here with a precise ID. The color pattern is so distinct that it ought to be easily identifiable. Any ideas?