Phenotypic induction in Pieris napi L.: role of temperature and photoperiod in a coastal California population

. 1. Californian Pieris napi have previously been reported as producing dark‐veined and light‐veined adults from diapausing and. non‐diapausing pupae respectively; the converse has not been reported. 2. A population of the coastal subspecies venosa from Monterey County was examined to determine whether pupal temperature exposure was involved in proximate control of seasonal phenotypes. 3. When reared under continuous light at 25°C this stock produced seventeen diapause pupae, thirteen of which have eclosed yielding typical dark‐veined adults, and thirty‐eight non‐diapause pupae. 4. Of fifteen non‐diapause pupae held in a dark box at 25°C, fourteen produced typical light‐veined adults and one produced a dark‐veined individual of intermediate phenotype. 5. Of twenty‐three non‐diapause pupae held in a dark box at 10°C for 24 days, twenty‐two produced dark‐veined butterflies of intermediate phenotype and one produced a light‐veined adult. 6. When the experiment was repeated with an inland, population of subspecies microstriata , which is normally univoltine and monophenic, the temperature effect on phenotype was still present but less pronounced. 7. The nature of phenotypic determination is reviewed with particular regard for the developmental environment of butterflies from diapaused pupae. The thermal environment of reactivating diapausers, rather than the diapause state itself, may determine adult phenotype.