A butterfly grows in four steps: egg, caterpillar, chrysalis, and adult butterfly. The big change from caterpillar to butterfly is called metamorphosis. Butterflies have complete metamorphosis. This means the young look completely different from the adults. A caterpillar spends its time eating and growing. A butterfly pupa is called a chrysalis. A cocoon is the silk case a moth caterpillar spins. A caterpillar fastens itself with silk to a stem or twig before it becomes a chrysalis. The silk is either a pad or a girdle. Inside the chrysalis, the caterpillar's body breaks down and is rebuilt as a butterfly.

Monarch caterpillars eat milkweed. Milkweed is poison to most animals. The poison stays in the monarch's body and makes it taste bad. A monarch's bright colors warn birds that it tastes bad. The viceroy butterfly copies the orange-and-black look of bad-tasting butterflies like the monarch and queen. A monarch caterpillar grows through five stages. These stages are called instars. It sheds its skin between them. It grows from a few millimeters long to about two inches (5 cm) before it makes its chrysalis.

A butterfly's body has three parts. They are the head, the middle (thorax) and the end (abdomen). A butterfly has four wings, two in front and two in back. A butterfly's wings are covered with tiny scales. A butterfly drinks with a long tongue called a proboscis. It curls up like a spiral when the butterfly is not drinking. A butterfly's large compound eyes let it see in almost every direction at once. A butterfly finds sugary food by touching it with its feet. The feet can sense dissolved sugar. A female butterfly drums a leaf with her feet. She tastes whether it is the right plant for her caterpillars.

In fall, many monarchs fly far away to a warmer place for the winter. Monarchs east of the Rocky Mountains spend winter in Mexico. Monarchs west of the Rockies spend winter in California. Some monarchs fly as far as 3,000 miles. They fly to reach their winter home. As of November 2026, the longest one-day flight of a tagged eastern monarch was 265 miles. Monarchs fly north in spring over three to five generations, because each generation lays eggs and then dies. Most adult monarchs live only a few weeks.

As of November 2026, there are far fewer monarchs now than there used to be. 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. Mexico protects the mountain forests where monarchs spend the winter in the Monarch Butterfly Biosphere Reserve.

Words to know

• metamorphosis: big change
• chrysalis: changing case
• instars: growth stages
• proboscis: long tongue