A caterpillar spends its time eating and growing. A caterpillar sheds its skin as it gets bigger. This is called molting. The big change from caterpillar to butterfly is called metamorphosis. Butterflies have complete metamorphosis. This means the young look completely different from the adults. 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. Adult insects do not grow, so a butterfly's adult size is set by when its caterpillar stops growing and starts to change.

North American monarchs form two groups. They are an eastern group and a western group, divided by the Rocky Mountains. The eastern group is much bigger than the western group. Monarchs east of the Rocky Mountains spend winter in Mexico. Monarchs west of the Rockies spend winter in California. 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. Monarchs fly north in spring over three to five generations, because each generation lays eggs and then dies.

Diapause is a long rest in growth or breeding that lets insects get through a hard season. Monarchs that migrate stop breeding in the fall. Monarchs in diapause can live for many months, up to about nine. 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 have fewer places to lay eggs because milkweed has been lost from many fields and roadsides. Forests where monarchs spend the winter are shrinking because of cutting of trees, storms and building. Weed killers kill the milkweed caterpillars need. They also kill the wildflowers adults feed on. Insect sprays can kill monarchs. People can help monarchs by planting milkweed in yards and gardens. The best milkweed to plant is the kind that grows naturally where you live. Mexico protects the mountain forests where monarchs spend the winter in the Monarch Butterfly Biosphere Reserve.

Words to know

• metamorphosis: big change
• imaginal: adult-building
• diapause: long rest
• generations: life-cycle rounds