Papilio thoas; Print; Papilio thoas; the king swallowtail or Thoas swallowtail; is a butterfly of the family Papilionidae. It is found in the southernmost United States; Mexico; Central America and South America (as far south as Argentina and Uruguay). The species is easily confused with the giant swallowtail; which it closely resembles in both larval and adult stages. The caterpillars feed on the leaves of citrus plants (Rutaceae). They have also been reported as feeding on a member of the genus Piper.