As autumn arrives in the United States and Canada, you could see the yearly migration of the monarch butterflies(帝王蝶).
No other butterfly in the world migrates like the monarch butterflies of North America. Every year, they fly to the south because they can't live for long periods in cold weather.
So each autumn around October, the butterflies travel up to 3, 000 kilometers south to spend winter in warmer locations. Some even fly to the mountains of Mexico. There are so many of them that they almost block out the sky. And you can hear their wings beating.
It usually takes the butterflies two months to reach Mexico. After five months of staying, they head back north in the spring.
But not one butterfly finishes the whole round-trip. They lay eggs along the way and die. Three generations(代)of butterflies will live and die during the spring journey alone. The fourth and final generation of the year is born in early autumn and will reach the north. But the butterflies hardly get to rest before starting the long journey south again to Mexico.
The most amazing thing about these butterflies is that they return to the very same trees in Mexico that their great-great-grandparents used to live on the winter before.
However, the population of this colorful butterfly has dropped in recent years. One of the main reasons, experts say, is that farmers have used herbicides(除草剂) on plants. The monarchs lay eggs along the route of their migration. Their larvae(幼虫) live and feed on the plants. But the herbicides can kill them.
