Science 325, 1700–1704 (2009)

Every autumn, monarch butterflies (Danaus plexippus) make a 4,000-kilometre journey from southern Canada to central Mexico. The butterflies navigate using the Sun together with an internal 'clock' to keep track of time and compensate for the Sun's shifting position in the sky.

Now, Steven Reppert of the University of Massachusetts Medical School in Worcester and his colleagues have discovered that this clock is housed in the antennae — not in the brain, as had long been assumed. When the researchers removed the monarchs' antennae, the insects flew in random directions. When the antennae were painted black to block out sunlight, the butterflies all flew in the same direction — just the wrong one. Monarchs whose antennae were coated with clear paint oriented themselves in the correct southerly direction.