How did dipping the antennae in paint help scientists discover that monarchs use their antennae to orient?

Scientists observed that the monarchs with black paint covering their antennae lost their ability to orient (tell direction). The monarchs painted with clear paint were not affected. (The butterflies with clear paint were in the control group of the experiment.)

Scientists knew the black paint was blocking light. Why do the monarchs' antennae need light? What is the role of the antennae in navigation?

• Read about the mysteries of monarch navigation in this week's slideshow!

Photo: Elizabeth Howard