Study species
Pararge aegeria , the speckled wood, is a woodland-associated satyrine butterfly found across Eurasia. Its life cycle is complex, strongly shaped by photoperiod, and varies geographically with local climate (Nylin et al., 1989, 1995). In Sweden, bivoltine populations (i.e. producing two generations per year) exist in the south and on some Baltic islands (Lindestad et al., 2019), while northern mainland populations are univoltine (one generation per year). Generally speaking, pupal diapause is induced by short days during the larval period, while larvae exposed to long days enter nondiapause development. By definition, more or less all individuals born in a univoltine population go through diapause, but northern P. aegeria can still be made to go through nondiapause development in the laboratory, showing that they are kept from doing so in the wild by local adaptation of photoperiod thresholds (Lindestad et al., 2019). Speckled wood larvae headed for pupal diapause tend to develop much slower, and (in bivoltine populations) reach a somewhat larger final size, than individuals not headed for diapause (Van Dyck & Wiklund, 2002; Aalberg Haugen et al., 2012; Aalberg Haugen & Gotthard, 2015). In addition to the nondiapause and pupal diapause pathways, P. aegeria is also capable of diapausing in the third larval instar; however, for the sake of simplicity, and because pupal diapause appears to be the dominant form of diapause in Scandinavia (Wiklund et al., 1983; Gotthard & Berger, 2010), larval diapause will not be considered here.