This is one of the most elaborate mimicries in the Lepidoptera because it isn't one image but a scene — not a fly, but two flies, and the thing they're eating, bird poop! Predators that avoid dropping-feeding insects on disease grounds skip it. The moth also releases a pungent smell, so the deception runs through two senses at once. The blue outline around the white patches is the part that gets me: it's a rendering of specular highlight, a painter's trick, arrived at by selection.