Once a year the cloud forest pine trees in the mountains of central Mexico take on spectacular raiment as they play host to millions of over-wintering monarch butterflies. Upon encountering the sea of orange and black wings that dress each tree from top to bottom, it is easy to understand why the Aztecs believed the monarchs were reincarnated warriors, resplendent in their battle colors. In fact, the life cycle of these butterflies is almost as fantastic, if only because they exhibit migratory behavior that is unparalleled in the insect world. When the weather begins to improve, the adult monarchs begin to fly north. As they migrate, they stop to lay eggs on milkweed plants. The caterpillars then eat the poisonous milkweed, utilizing its toxins as a defense against predators. The caterpillars grow and pupate, emerging as adults that continue the northerly migration.
Toward the end of summer, reduced temperatures and daylight trigger behavioral changes in the adults, causing them to begin to fly south once again. Fat stored in the abdomen allows the butterflies to make the long journey to Mexico, which can be as far as 3,000 miles (4,800 kilometers). Scientists are still not certain how monarch butterflies navigate, because each individual only manages to complete part of the round trip during its short lifetime. The hundreds of millions of butterflies that create the wonderful spectacle are the great-great-grand-offspring of the ones from the year before.