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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, the researchers say.

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 the study's co-author.

The scientists discovered that the part of the butterfly's 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.

--Los Angeles Times