On overcast days, monarch butterflies use a magnetic compass to find their way south, making them one of only a few migratory insects known to sense Earth's magnetic field.

The eastern North American monarch butterflies (Danaus plexippus; pictured) use the Sun to guide them from southern Canada and the United States towards Mexico, but they still manage to fly in the correct direction on cloudy days. Steven Reppert at the University of Massachusetts Medical School in Worcester and his team studied butterflies in a flight simulator inside an artificial magnetic field. The authors found that the butterflies changed their flight orientation when the researchers shifted the magnetic field, but only when the insects were exposed to certain wavelengths of light.