A key adaptation in insects for dealing with variable environmental conditions is the ability to diapause.The tiger swallowtail butterflies,Papilio glaucus and P canadensis are ideal species to explore the genetic causes and population genetic consequences of diapause because divergence in this trait is believed to be a salient factor in maintaining a hybrid zone between these species.Yet little is known about the factors that influence diapause induction in this system.Here we explored how spatial (latitudinal),environmental (temperature) and genetic (hybridization) factors affect diapause induction in this system.Specifically,a series of growth chamber experiments using wild caught individuals from across the easte United States were performed to:(1) evaluate how critical photoperiod varies with latitude,(2) isolate the stage in which induction occurs,(3) test whether changes in temperature affected rates of diapause induction,and (4) explore how the incidence of diapause is affected in hybrid offspring.We find that induction occurs in the larval stage,is not sensitive to a relatively broad range of temperatures,appears to have a complex genetic basis (i.e.,is not simply a dominant trait following a Mendelian inheritance patte) and that the critical photoperiod increases by 0.4 h with each increasing degree in latitude.This work deepens our understanding of how spatial,environmental and genetic variation influences a key seasonal adaptation (diapause induction) in a well-developed ecological model system and will make possible future studies that explore how climatic variation affects the population dynamics and genetics of this system.