It is spring and the Monarch Butterflies are well into their northward migration. Monarchs find their way north by following their sun compass. Polarized light receptors in the dorsal rim of the compound eye can accurately detect the direction of the sun. Light is most strongly polarized in the direction of the sun. During the fall, Monarchs are conditioned to use their sun compass to fly toward the direction of the sun which leads them south to overwintering sites in Mexico. During the winter in Mexico, Monarchs are exposed to cold temperatures which resets their behavior. Instead of flying toward the sun, Monarchs fly in the opposite direction of the sun which leads them north.
During the day, the sun moves from east to west across the sky. Flying directly away from the sun is only true north at noon. Monarch Butterflies also have a circadian clock in their antennae. The clock tells the Monarch whether it is morning, noon or evening. Monarch adjust their flight relative to the sun to compensate for time of day.
The current generation of Monarchs, larvae that are now feeding on milkweed in the southern US, will pupate, emerge as adults and follow their sun compass north. We anticipate the arrival of the first Monarch adults in Indiana late in this month of May.