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If migrating birds are awe-inspiring, monarch butterflies that make the long perilous trek from Canada to the mountains of Michocan, Mexico, are dumbfounding. This is especially true when one realizes that the butterflies that arrive like a blizzard in November are at least three generations removed from the butterflies that left these same forests the preceding March.

How do they do it? Scientists don't really know. But part of the mystery may have been solved recently by entomologists at the University of Kansas-Lawrence and the University of Arizona-Tucson. They claim the butterflies carry their own internal compasses. When monarchs captured during fall migration were exposed to normal magnetic fields, they flew southwest - as if to Mexico. When magnetic fields were reversed, they flew as if returning from whence they came. And when exposed to no magnetic field, they flew in every direction.

That still doesn't explain how they happen to congregate by the millions in the same Mexican mountain-top forests every year. Some researchers believe it's a genetic "imprint" passed on from their great- and great-great grandparents.

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