Every winter Monarch butterflies head south to Mexico to avoid cold temperatures.

Want to learn more about Monarchs and other butterflies? Visit the Science Museum's Monarchs and Migration website

But how in the world do they know how to get there? Well, they don't follow Highway 35, that's for sure. It turns out that monarchs can detect the sun's ultraviolet (UV) rays even when it's cloudy out. (UV rays are the part of sunlight that causes sunburn.)

Up until now we didn't know how butterflies used this UV information to fly south. Researchers led by Steven Reppert at the University of Massachusetts Medical School ran some monarchs through a flight simulator and discovered their secrets. It turns out that monarchs' eyes are very sensitive to UV light. They synch this UV information up with a natural clock in their brain. By combining these two bits of information, monarchs are able to determine the angle of the sun and always head due south. Sailors used a similar method (a sextant) to navigate around the world before the invention of compasses. Monarchs can do the trick all by themselves, though.

Do you think you could walk due south, from Minnesota to Mexico even on a cloudy day?

Some tagged monarchs have travelled more than 265 miles in a single day! Not bad for an insect...

Journey North and Monarchs in the Classroom also have cool websites (complete with projects and "Citizen Science" opportunities) about the annual Monarch butterfly migration.