Monarch butterflies making their annual migration from the eastern United States to winter residences in Mexico's Sierra Madre mountain range find their way by following a three-dimensional map made of rays of polarized ultraviolet light, according to a new study.

Though UV light is invisible to humans, to butterflies it appears as a grid in the sky that emanates from the sun, researchers reported last week in the journal Neuron.

As the sun travels from east to west across the sky, so does the grid. To compensate, the butterflies use an internal clock that recalibrates the grid throughout the day so they can travel in a straight line, said Dr. Steven Reppert, a professor of neurobiology at the University of Massachusetts Medical School and co-author of the study.

Reppert and his colleagues knew the butterflies used polarized light to navigate, but they weren't sure it was from the ultraviolet portion of the spectrum. Their suspicions were confirmed when they put the insects in a barrel-sized flight simulator and used a plastic filter to block UV light. The butterflies still could see, but they flew around in circles.

"Without [UV], they get very confused and lose their sense of direction," said co-author Adriana Briscoe, an assistant professor at University of California, Irvine, who studies butterfly vision.

The scientists discovered that the part of the butterfly visual system that detects polarized light is dominated by photoreceptors for UV. To their surprise, they also found that those receptors are linked to neural fibers that contain a key protein used to regulate the butterfly's internal clock.

"If you want to understand the genetic basis of migration, you have to have some idea of how the information gets into the brain in the first place," Briscoe said.