A Clock Inside the Antenna
The monarch butterfly does not simply fly toward the sun. That would leave it hopelessly off course as the sun arcs across the sky from east to west. Instead, the butterfly compensates for the sun's movement by running it against an internal clock, a circadian timer housed not in the brain but in the antennae. Scientists at the University of Massachusetts Medical School, publishing in Science in 2009, showed that monarchs with painted antennae (which blocked light input to the antennal clock) flew in random directions, while intact butterflies held a consistent southwest bearing. Remove the antenna, and the compass breaks. The antenna is not just a sensory organ. It is the timepiece the sun-compass depends on.
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