The Monarch butterfly (Danaus plexippus) is a milkweed butterfly and perhaps the best known of all North American butterflies (though it sometimes wanders to Oceania and Europe). Every year the population east of the Rocky Mountains goes on a southward migration and northward return, the only butterfly species to do so. Females, such as the one pictured here, will lay eggs during the trip; the migration spans the life of three to four generations.Photograph: Kenneth Dwain Harrelson