In late autumn, most Monarch Butterflies are well on their way to overwintering sites. Monarch Butterflies have multiple systems to help them navigate the length of North America. These systems can detect earth’s magnetic field, the direction of the sun, the time of day, wind speed, wind direction and other factors that lets migrating butterflies arrive at an appropriate site. A sun compass is one of the systems a Monarch uses to navigate. In late autumn, the sun is low in the sky, but the sun is only in the direction of “True South” at noon. A clock is needed to navigate with a sun compass and adjust the flight direction relative to the position of the sun.
Monarch Butterflies and other insects have multiple circadian clocks. These clocks depend on feedback from light sensitive chemicals that peak at dawn and are lowest at dusk. Like most insects, the Monarch Brain has an area that serves as a circadian clock. This clock in the brain was assumed to be used in sun compass navigation but never tested. Evidence that many insects have a circadian clock located in the antennae led to the investigation of Monarch Antennae. Merlin and colleagues* found a circadian clock in the antennae of the Monarch and demonstrated that disrupting the antennal clock affected the ability of Monarchs to navigate.
Insect antennae can provide insects with the time of day in addition to much other information.
*Christine Merlin, Robert J. Gegear & Steven M. Reppert. 2009. Antennal Circadian Clocks Coordinate Sun Compass Orientation in Migratory Monarch Butterflies. Science 25 September 2009: Vol. 325 no. 5948 pp. 1700-1704.