A butterfly that weighs less than a paper clip makes one of the longest trips in the insect world.
The Monarch Butterfly (Danaus plexippus) does something that sounds impossible. Every fall, Monarchs from the United States and Canada fly south to spend the winter in a mountain forest in central Mexico. The trip can be 3,000 miles. And here is the amazing part: none of them have ever been there before.
It works like this. Most Monarchs live only about six weeks. Through spring and summer, three or four short-lived generations are born, one after another. But the generation born in late summer is different. These butterflies live about eight months. Scientists sometimes call them the super generation. They are the ones that make the whole trip south.
They fly over the Great Lakes, across the prairie, over the Rio Grande, and up into the mountains of Mexico. In November they arrive at a small group of fir forests about 10,000 feet up. It is the same few mountaintops every year. The butterflies that wintered there last year were their great-great-grandparents. No living Monarch has made the trip before or met one that has. They find it anyway.
How? Scientists know part of the answer. Monarchs steer by the position of the sun, and a built-in clock in their antennae helps them correct for the time of day. They can also sense the Earth's magnetic field. But nobody fully understands how all the pieces add up to finding one small forest on a map they have never seen.
Monarchs depend on milkweed. It is the only plant their caterpillars eat, and it makes them poisonous to birds. If you plant milkweed, you are helping the next super generation get where it is going.