Fig. 1.

Life cycles of (A) a bivoltine solitary wasp and (B) Polistes in a seasonal environment. Eggs of the first brood (G1) are laid by adults from the second generation of the preceding favorable season (G2). In the solitary wasp, G1 females complete development, emerge as adults, and produce the G2 generation. These individuals then pass the unfavorable season, indicated by shading between the dotted vertical lines, in prepupal diapause (35). Three principal changes can convert this solitary life cycle into a social life strategy. One change is that diapause is passed as an adult rather than as a prepupa, which is known to occur in some bivoltine solitary wasps in family Sphecidae (35). A second change is that the life cycle is partially rather than discretely bivoltine. In partial bivoltinism, adults that lay eggs of the first generation also lay some eggs of the second generation. This strategy is common and may favor evolution of eusociality (6). The third change is that offspring of the first generation do not reproduce but instead undertake brood care at their natal nest; thus, no arrow connects the G1 and G2 broods in the Polistes diagram (B).