Regulation of hatching year by seasonal timing of oviposition in the stick insect, Phraortes elongatus (Phasmatodea: Phasmatidae)

Abstract

The effects of oviposition season on embryonic development and hatching in Phraortes elongatus (Thunberg) (Phasmatodea: Phasmatidae) were studied. Nymphs collected from spring through early summer in Okayama, Japan, were reared under outdoor conditions or under constant conditions in the laboratory. After adult emergence, their eggs were used for experimentation. Eggs laid in early summer hatched in the spring of the following year, whereas those laid in autumn hatched in the spring of the second year after oviposition. Dissection revealed that eggs laid in early summer had completed their embryonic development before the first winter, while those laid in autumn were in the early embryonic development stage. Eggs that did not hatch during the first spring completed their embryonic development during autumn. These findings suggest that P. elongatus eggs enter diapause to pass the first winter in an early or late embryonic developmental stage depending on the timing of oviposition. If eggs pass the first winter in an early embryonic developmental stage, diapause is also induced before the second winter in a late embryonic stage. At a constant 25 °C, most eggs completed embryonic development 8 weeks after oviposition and entered diapause. This suggests that warm temperatures during early embryonic development avert the diapause of early-stage embryos. Thus, P. elongatus has a univoltine life cycle when eggs are laid early in the reproductive season, and semivoltine when laid late.