Clear as Mud by Paul Jennings

"Clear as Mud", in which the protagonist steals a rare insect from a lab, gets bitten by it, and soon finds that his skin is turning invisible to reveal his bones and innards, just like an X-Ray. He flees a terrified but fascinated public, and lives as a desert hermit for over ten years, keeping the beetle in a glass jar. Then it bites him again and his skin is restored. However, there's a rather horrible twist