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.