Monarch butterflies live in North, Central, and South America as well as Australia, some Pacific Islands, India, and Western Europe. Their markings include bright orange wings covered with black veins and rimmed with a black border and white dots. Females have thicker veins in their wings. A monarch's brilliant coloring tells predators: "Don't eat me. I'm poisonous." The butterflies get their toxins from a plant called milkweed, which is their only food source in the caterpillar stage. An animal that eats a monarch butterfly usually doesn't die, but it feels sick enough to avoid monarchs in the future.
The most amazing thing about monarch butterflies is the enormous migration that North American monarchs undertake each year. Every fall, as cold weather approaches, millions of these delicate insects leave their home range in Canada and the United States and begin flying south. They continue until they reach Southern California or central Mexico, more than 2,000 miles (3,200 kilometers) away!
These international travelers return to the same forests each year, and some even find the same tree that their ancestors landed on. Some estimates say up to a billion butterflies arrive in the mountains of Mexico each year.
Scientists aren't sure how migrating monarchs know which way to go, since they only live a few months and none makes the journey more than once. Toward the end of winter, the monarchs in Mexico and California mate. The males then die, while the females head north, depositing eggs on milkweed plants along the way and eventually dying themselves. From these tiny, round eggs come small green-and-white-striped caterpillars, which feed on the milkweed leaves. For about two weeks, they eat constantly and grow by shedding their skin. They are then 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 and slowly becomes white, then see-through. 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. It must wait for its wings to dry before it flies away. Adult butterflies don't grow. They survive by drinking nectar from flowers, including milkweed, clover, and goldenrod.
Scientists think North American monarchs have been making their amazing annual journey for thousands of years. But logging in Mexico has greatly reduced forests where these butterflies roost. Efforts to protect these lands are helping, but illegal logging still takes a toll. Since 1983, the International Union for Conservation of Nature has listed the monarch migration as a "threatened phenomena."