Monarch butterflies have bright orange wings covered with black veins and rimmed with a black border and white dots. This brilliant coloring tells predators: "Don't eat me. I'm poisonous." They get their toxins from milkweed, which is their only food source in the caterpillar stage. An animal that eats a monarch butterfly usually doesn't die but does feel sick enough to avoid monarchs in the future.
Each year, North American monarchs undertake a migration of nearly 2,500 miles. In the fall, millions of them leave their summer home in Canada and the United States and fly south. Western monarchs (found west of the Rocky Mountains) spend the winter on the California coast. Eastern monarchs (found in the eastern United States and Canada) spend the winter in Mexico’s fir tree forests. Scientists think North American monarchs have been making their annual journey for thousands of years. They return to the same forests each year, and some even find the same tree that their ancestors landed upon. Experts say up to a billion butterflies arrive in the mountains of Mexico each year.
Toward the end of winter, monarchs mate and then the males die. The females head north, depositing eggs on milkweed plants along the way and eventually dying themselves. From these tiny, round eggs emerge small green-and-white- striped caterpillars. They eat feed constantly on milkweed leaves for about two weeks and then are ready to transform into pupae. To become a pupa, also called a chrysalis, a monarch larva attaches itself with silk to a leaf or branch, sheds its skin, and forms a hard shell. This vase-shaped case starts out green with shiny golden dots, then slowly becomes white, and finally transparent. After 9 to 15 days, a fully formed butterfly emerges.
The entire egg-to-butterfly process, called metamorphosis, takes about a month. Once out of the pupa, the damp butterfly inflates its wings with blood stored in its abdomen, waits for its wings to dry, and flies away.