To study these abilities, Reppert's team decided to confuse the butterflies, at least a little, by removing their antenna. But this did more than eliminate their sense of smell and vibration. It totally befuddled their navigation.

Reppert says, "This finding was actually quite heretical to us. We did not know quite what to do with it at the time." So, they devised another experiment.

Dr. Thomas Barnes/USFWS Removing the Monarchs' antenna not only eliminated their sense of smell and vibration, it also befuddled their navigation.

On the chance that smell mattered more than they had realized, the researchers painted butterflies' antenna to block their odor receptors. And just in case sunlight played a role, Reppert's team used clear paint on some butterflies' antenna and black paint on others. The clear paint allowed in daylight, and those butterflies navigated normally. The ones painted black could not.

Antenna data leads the way

This meant that the Monarch's antenna can detect light. But an even bigger surprise was up ahead. Scientists knew that the circadian clock in the insect's brain responds to light signals from the eyes. They had assumed that that system provided all the daylight data that the butterfly's navigation system needed.

But it turns out that the circadian clock signals produced in the butterfly's antenna are at least as crucial as the clock in its brain. The butterfly's bug-sized GPS needs that antenna data, so when winds and other surprises hit, the Monarch still flies in the right direction.