Seasonal Adaptation vs Physiological constraint: Photoperiod, Thermoregulation and Flight in Colias Butterflies

Photoperiod is already a known cue for the seasonal (spring and autumn) wing underside darkening, a polyphenism which is adaptive by increasing solar absorptivity, of the North American butterfly, Colias eurytheme. We now report that photoperiod is also a cue for seasonal (spring and autumn) deepening of thoracic «fur» (actually modified scales), which increases the insects'insulation against convective cooling, and thus is likewise adaptive to flight in these cooler seasons. Conversely, the less absorptive and less insulated summer forms are more adaptive in warm conditions because of lesser susceptibility to dangerous overheating. 2. Changes in absorptivity and fur thickness cued by photoperiod are in an uncorrelated fashion