Larval development in a northern population of the Western Palaearctic P. nymphula has been studied in the field and laboratory to identify responses to physical factors that regulate the time of its annual emergence. In the field, as in more southerly British populations, development is normally semivoltine. Typically, larvae overwinter first in a range of early instars and enter the penultimate (F-l) instar in their second summer, remaining in that instar until late August, after which they enter the final (F-0) instar rapidly during the next month and overwinter in that instar. Arrest of the senior age-cohort in F-l during summer and then in F-0 during the winter reduces temporal variation among larvae and contributes greatly to synchronisation of metamorphosis and then emergence in early summer (May to early July). A variable, usually small, proportion of the senior cohort spends the second winter in F-1. In spring such larvae enter F-0 rapidly in May and proceed directly to metamorphosis and emergence. Such individuals are smaller, as F-0 and as adults, than those that overwintered as F-0 and they constitute a second, later component of the emerging population.