The Monarch Mystery  PRO

The Monarch Mystery.

The ability to find home evokes legends of Rover or Fido who, when owners have moved from one coast to the other, have made a 3,000-mile trek to find their owners in a location to which they’ve never been before. At least the dogs make it back home.

But not the monarch butterfly. These insects somehow know how to migrate thousands of miles every autumn, from the Eastern United States to a handful of sites in Mexico. There, they rest over the winter for the return trip home. But here’s the amazing part: No individual butterfly ever goes to Mexico and back, yet thousands converge on the same few sites year after year. These insects know where to go. But none of them has ever been there before. Let’s explain.

"Monarchs are not guided by memory, since no single butterfly ever makes the round trip. Three or four generations separate those that spend one winter in Mexico from those that go there the next." A monarch butterfly born in August in New York state, for instance, will fly all the way to Mexico, spend the winter there, and leave in March. Then it will fly north, laying eggs on milkweed along the Gulf Coast in Texas and Florida before dying.

The butterflies...