By Stephen Day

How does the monarch butterfly (Danaus plexippus)navigate the so-called “last mile,” or final stretch, of its migration? The precision with which these little insects home in on their target destination is nothing short of astounding. The question has foxed entomologists over the years.Scientists agree that monarchs use a complex group of sensors, mainly on their antennae, to determine when and where to migrate. Many years ago, I spoke with an entomologist specializing in monarch studies. I asked him what exactly would happen to the butterflies’ migration capabilities if their antennae were removed one at a time. “Well, of course, they couldn’t migrate, feed on nectar [or] lay their eggs on milkweed plants to reproduce. It’s their antennae, stupid,” he answered brazenly.

Scientists have since peeled the proverbial onion and located light and temperature sensors in a monarch’s antenna, which recognize the time of day and the sun’s position in the sky. Another section of the brain comprising iron molecules uses this information to act as a GPS-like magnetic compass. These all are connected to the monarch’s stereoscopic antennae sensors. These functions explain how the East and West Coast monarch species know when to initiate migration, and how they can find their cardinal directions, and specific roosting sites.

Now the question becomes: How do East Coast monarchs manage to migrate........

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