The most extensive work on insect obesity has been done on fruit flies. Larvae fed high-calorie diets tend to fatten up quickly, though ones with high-sugar diets develop a condition similar to diabetes and suffer shortened lifespans. Once a fly reaches adulthood, though, there’s a limit to how big it can get. Just like a human, the fruit fly stores its excess energy as lipid droplets, which are encased in cells. (Our lipid droplets live in fat tissue; a fruit fly has a comparable organ called the “fat body.”) But grown-up flies, like other insects, are encased in a chitin exoskeleton. That means their bellies can’t expand, says Thomas J. Baranski, an endocrinologist at Washington University. “Because it’s got this exoskeleton, it just packs the fat in tighter.”