Monarch butterflies emerge from their chrysalis during a short time window starting just before dawn and with a peak emergence shortly after daylight. Long before noon, each day’s cohort of adults will have emerged. The next monarchs will not emerge until just before dawn the next day. The advantages of early morning eclosion are not fully understood. However, a newly emerged monarch can expand its wings and fly off to feed on nectar as soon as its wings are fully expanded and dried.Like most insects, monarchs have an internal circadian rhythm that will keep time even in the absence of light. If a monarch chrysalis is entrained to a light dark cycle, then placed in complete darkness, the monarchs will emerge in complete darkness during the same time of day, as the monarchs that are kept on a light-dark cycle.
The monarch circadian clock uses a protein called “period”, the same protein that is used in the Drosophila fly clock. “Period” is regulated so its concentrations in the brain build up to a high level, then crash to nothing during a cycle that runs about 24 hours. The level of “period” in the brain tells the monarch what time of day it is.
All clocks need a mechanism to set the correct time. In monarchs, the concentration of “period” can be reset by exposure to light. In a normal light-dark cycle, the clock is reset when the sun goes down. From that moment forward, the clock will run by itself and keep accurate time, even in total darkness.