Migratory species may display striking phenotypic plasticity during individual lifetimes. This may include differential investment in body parts and functions, differential resource use and allocation, and behavioural changes between migratory and non-migratory phases. While migration-related phenotypic changes are well-reported, their underlying mechanisms are usually poorly understood. Here we compare individuals from migratory (reproductive diapause) and non-migratory (reproductive) phases of closely related aposematic butterfly species to study how sexual dimorphism and migratory behaviour underlie significant morphological...