Larva are horizontally striped with black, white and yellow. Pupae are a brilliant green colour, with a gold band near their silk point of attachment to a leaf or branch. The adult monarch is orange with black stripes radiating from the point of attachment of the wings to the thorax. The black edges of the wings are dotted with white spots. Males are distinguishable from females by the presence of black coloured scent glands on each of their hind wings.
Monarchs breed in the northern United States and southern Canada.
Western populations of adults, which emerge late in the season, migrate to California, and eastern populations migrate to the Sierra Madre mountains in Mexico to overwinter. A monarch's bright colors may well be an advertisment to predators of its bad taste, since a butterfly caught and spit out may be fatally injured. In addition, viceroy butterflies, which are unrelated to monarchs but look almost exactly like them, are not bad tasting to birds, but may have evolved to mimic monarchs and thereby avoid the same kind of predation. The mystery of monarch migration was solved by the use of lightweight wing tags, which directed people who found tagged monarchs to send them to Dr. Fred Urquhart at the University of Toronto. Records of tagged butterflies were then tracked and their migratory route determined.