Organisms deal with the opportunities offered and the problems posed by the environment through their phenotype, including their morphology, physiology, behaviour and life history traits. Phenotypic plasticity (i.e. the property of a genotype to produce different phenotypes in response to different environmental conditions) allows them to respond to environmental change like seasonal variation. Short-living animals with more than one generation a year may show different phenotypes in different seasons. We study one of the most striking examples of seasonal plasticity among temperate-zone butterflies: i.e. in the Map butterfly (Araschnia levana). Proximate factors affecting the development of the two forms are well known, but our project aims for testing ultimate explanations for the assumed adaptive nature of the seasonal forms (e.g., relative to anti-predation and thermoregulation).