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Every year, millions of monarch¹ butterflies fly thousands of miles to alight in one specific forest in Mexico. How do they know what direction to flutter? Scientists had thought that an insect GPS system in their brains steered² them in the right direction.

Now, a new study published in the journal Science overturns that idea. Because sun-related directional sensing actually resides in the butterfly’s antennae³, say scientists at the University of Massachusetts Medical School.

The researchers remembered a half-century old study that noted⁴ that if butterflies lost their antennae mid-flight, they became disoriented. So they put butterflies in a flight simulator and tried to convince them to fly south. Those with their antennae intact had no problem orienting and flying south. But those without their antennae just couldn’t do it. Next they painted some butterflies’ antennae black, blocking light sensing. Those insects couldn’t orient themselves. But when researchers covered antennae in clear paint, the butterflies could once again fly in the correct direction.

Butterfly antennae were already known to sense odor, wind, even sound. Now it seems that they’re also vital for getting lonely Lepidopterans back to Mexico to mingle—and make the next generation of monarchs⁵.