Without ultraviolet light, a monarch butterfly will get disoriented. This was a major finding in a study of how the insects navigate during their 3000-mile migration. Because each creature’s trek is a once-in-a-lifetime trip, there are no guides who teach younger members of the species the route. A group that was led by Dr. Steven Reppert of the University of Massachusetts Medical School in Worcester and that also included colleagues from the Hebrew University of Jerusalem, the Academy of Sciences of the Czech Republic in Prague and the University of California, Irvine, studied butterflies’ brain and eye tissues. The scientists found that light in general triggers the biological clock mechanism in the insects, giving them the impetus to fly, but that it is UV radiation specifically that keeps them oriented. When UV wavelengths were filtered out, butterflies tested in a flight simulator lost their sense of direction. Further investigation showed that the UV receptors in the butterfly’s eye are connected to the biological timing regions of the brain. So, for a monarch in migration, UV is king of the road.