Millions of monarch butterflies fly southwest from eastern Canada and the United States down to Mexico each autumn; then millions more fly back to the northeast in the spring. The one-way trip is as long as 2500 miles for some of these creatures. The question for this Moment of Science then is how do the butterflies know what direction to fly?
Previous studies have shown that the butterflies can use the sun to orient themselves in the right direction; a more recent study has shown that they can also sense direction from the Earth's magnetic field. To prove this, researchers collected monarch butterflies in the midst of their migration to Mexico and placed them in a room unshielded from the Earth's magnetic field. Sure enough, the butterflies traveled southwest, the same direction they fly in their migration.
Then researchers placed the butterflies in a room shielded from the magnetic field. In result, the butterflies flew every which way in confusion. Researchers then tried a third test in which they exposed the butterflies to a magnetic field the opposite of the Earth's. As expected, the butterflies flew in the opposite direction, toward the northeast, suggesting that they are indeed guided by the magnetic field.
Monarch butterflies are hardly the only creatures to use magnetic orientation, though. Among other creatures that possess this device are honeybees, sea turtles, some wasps, some fish, and all migratory birds ever tested.