Groups of cells called imaginal discs survive inside the chrysalis and use the broken-down caterpillar as fuel to build the wings, eyes, antennae and legs of the adult. Not everything in the caterpillar dissolves in the chrysalis; some muscles, parts of the nervous system, the breathing tubes and the gut carry on into the adult. Hormones, not the caterpillar's choice, steer growth and change, and juvenile hormone keeps a young insect from starting metamorphosis too soon. Adult insects do not grow, so a butterfly's adult size is set by when its caterpillar stops growing and starts to change. A study suggests that moths remember what they learned as caterpillars: experiments with shocked caterpillars found that adults remembered the smell linked to the shock.

The sun moves across the sky but the monarch's goal stays put, so monarchs use an inner clock to correct their sun compass for the time of day, and the timing clocks that correct the monarch's sun compass are in its antennae, not only in its brain. Butterflies can sense polarized light, a pattern in skylight that people cannot see, and monarchs may use it with the sun to steer. Flight-simulator tests show that migrating monarchs have a magnetic compass that senses the tilt of Earth's magnetic field, and it is probably a backup for days when the sun is hidden. A monarch's sense of direction for the trip south is inherited, not learned, because each migrant is on its first trip.

Diapause is a long rest in growth or breeding that lets insects get through a hard season, and monarchs that migrate stop breeding in the fall. Monarchs in diapause can live for many months, up to about nine, while summer adults breed for only two to five weeks. The oyamel fir forests of Mexico stay cool but above freezing, about 0 to 15 degrees Celsius, so monarchs use little of their fat while they wait out winter. In early spring, monarchs wake from diapause at the winter sites, mate, and then spread north. Monarchs fly north in spring over three to five generations, because each generation lays eggs and then dies, and a painted lady round trip between Africa and Europe takes several generations, so no single butterfly makes the whole trip.

Scientists do not count each eastern monarch; they estimate the population by measuring the area of Mexican forest covered by roosting monarchs, in hectares. As of November 2026, a hectare of roosting monarchs holds an estimated 21 million butterflies, though the number varies from year to year. As of November 2026, in winter 2025-26, eastern monarchs covered 2.93 hectares (about 7.24 acres) of forest, up about 64 percent from the winter before. As of November 2026, western monarchs have fallen by more than 95 percent since the 1980s. As of November 2026, eastern monarchs have fallen by 80 percent or more since the 1990s. In December 2024 the U.S. Fish and Wildlife Service proposed to list the monarch as threatened under the Endangered Species Act; as of November 2026 it is a proposal.

The line between butterflies and moths is fuzzy; some moths fly by day and some are brightly colored. Butterfly antennae have a club at the tip, and moth antennae are feathery or come to a point. The viceroy butterfly copies the orange-and-black look of bad-tasting butterflies like the monarch and queen, and scientists once thought the viceroy was only a harmless copy of bad-tasting butterflies. A study found that viceroys in northern Florida, where queens are rare, have become bad-tasting too. Some butterfly colors come from tiny structures in the scales that bend and reflect light, and not from pigment; the blue of a Blue Morpho is one example. The ridges on Blue Morpho scales are stacks of thin layers that reflect blue light strongly, like a tiny mirror tuned to blue.

Words to know

• polarized: lined-up light
• inherited: passed down
• hectares: land areas
• migrant: traveling animal