Monarch butterfly, or milkweed butterfly, migratory butterfly that is believed occasionally to fly across the Atlantic Ocean. It has brownish, orange wings with black veins and borders. Its caterpillars feed on milkweed, which gives their bodies disliked by birds. The unrelated viceroy butterfly mimics the colouration of the monarch and so is also avoided by birds. Length: 3 to 4.5 inches. Family Danaidae; species Danaus plexippus.