The monarch is sometimes called the "milkweed butterfly" because of the importance of this plant to its survival and development. Throughout most of the monarch's life, milkweed provides most of its food and a home. The toxins it takes in from the milkweed it eats makes it poisonous to predators.
Like other butterflies, monarchs develop through several forms after they hatch from their eggs: caterpillar (also called larva), chrysalis (also called pupa), adult butterfly. Eggs hatch after three to five days.
One egg produces one caterpillar, which will grow to about 5 cm (2 in.) long in about two weeks. For the last of the five times it will shed its skin, it attaches itself head-down to a convenient twig. Next, it sheds its outer skin and begins the transformation into a chrysalis. This process is completed in a matter of hours. Packed tightly inside the chrysalis, the caterpillar transforms into a butterfly in about two weeks. Adults live from three weeks to seven months, depending of the generation and the season.
There are two geographically distinct monarch populations in North America. The eastern population breeds east of the Rocky Mountains and migrates to central Mexico, where it spends the winter in patches of fir forest high in the mountains. Millions and millions of monarchs fly up to 4000 km (2,480 mi.) to get there. Until 1975, when the scientific community finally tracked down the wintering sites of the monarch in Mexico, the monarchs' winter hideouts had been a secret known only to local villagers and landowners.
The western population breeds in areas west of the Rockies and overwinters along the California coast. West of the Rockies, monarchs migrate to groves of trees along the coast of California.
Monarchs face threats to their survival. In Canada and the United States, large scale agriculture and herbicides are turning flower filled meadows into flower less monocultures. Without this summer habitat, monarchs will be unable to breed or migrate successfully. In Mexico and California, monarchs are even more at risk. Despite conservation efforts, tree cutting and development are destroying their winter homes. Because so many butterflies gather in only a few locations for the winter, this loss of habitat is devastating.