Figure 1.

Timing insect migration: from flight orientation to photoperiod-induced migratory switch. (a) Autumn migratory routes of western and eastern monarch butterflies on each side of the Rocky Mountains (brown lines), from their breeding to their overwintering sites in Mexico and California (light blue circle and dark blue line, respectively); (modified from Reppert et al. [22]). (b) Circadian clock control of monarch migration. Left, Circadian clocks in the antennae allow autumn migrants to time compensate their sun compass orientation to maintain a constant flight bearing to the south. The black arrows denote the southward orientation taken by migrants at any time of the day, and the grey dashed arrows denote the default direction the monarch would take in absence of time compensation. Right, Circadian clocks or clock genes in the brain may be part of a photoperiodic timer sensing a photoperiodic decrease that coincides with the onset of autumn southward movement. The blue dots depict the average latitude of eastern adult monarchs sighted at a given day between 1997 and 2013 (observations from Journey North, http://www.learner.org/jnorth/; sightings include the ‘First monarch butterfly’ and ‘Monarch first migration sighting’ for August–November; sightings from Florida were removed as they may be from a resident population). The orange dots depict the average photoperiod experienced by sighted adult monarchs at a given day.