I LOVE photo's like this where you get to properly see animals that you'd never get the chance to get close to in real life.
Firstly the focusing, absolutely superb, highlighting his eyes and body. Also part of his wings, showing the detail in them and the gorgeous patterns at the very back. Secondly, I love how natural this shot is. Too many times I see beautiful photos of butterflies ruined by effects and over photoshopping. It's a real treat to see one just as nature intended. There really is nothing to critique this photo on as it's just simply stunning, though if I had to be REALLY picky, his left antennae being out of focus is a little distracting, though as I say, that's being seriously picky.