Charles, Tobacco Hornworms eat plants in the nightshade family. So tomatoes, brugmansia, datura, etc. Caterpillars are pretty picky and only eat plants which can successfully host them. That's why we call them "host plants". So the caterpillar went back to the Datura because that's likely the only thing (or closest thing) it could eat. They will starve rather than eat the "wrong" plant.
