Like all butterflies, moths, and other insects with a lifecycle that involves complete metamorphosis, individual monarch butterflies (Danaus plexippus) go through four stages from eggs to larvae (caterpillars) to pupae to adults. Unlike other insects, as a species monarchs also undergo an annual cycle that involves several generations and a migration that covers thousands of miles.

Monarch butterflies are native to North and South America, but have were spread by humans throughout much of the world in the 1800s. They first appeared in Hawaii in the 1840s, then spread throughout the rest of the South Pacific in the 1850s and 1860s. In the early 1870s, the first monarchs were reported in Australia and New Zealand. These different populations have adapted to their new habitats with an amazing range of behaviors, but the annual migration undergone by the North American monarchs makes them unique among insects.

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