Adaptive advantages and the evolution of colony formation in Cyrtophora (Araneae

Cyrtophora moluccensis (Doleschall) in New Guinea and C. citricola Forskal in the Mediterranean and Africa form persistent aggregations of individuals of all ages. Characteristic of this “communal” organization (as defined by Wilson, 1971) in C. moluccensis are low level aggressive interactions during prey capture and during web repair and renewal. Such interactions may serve as a spacing mechanism, ensuring an adequate fool supply for each individual Disadvantages resulting from conspicuousness and persistence of C. moluccensis colonies are a high incidence of parasitism and a large degree of colony avoidance by flying insects (potential prey). Advantages of communal organization postulated for C. moluccensis includes increased protection of egg‐cases and young, greater web building efficiency, greater prey capture efficiency, and the ability to exploit habitats that are unavailable to solitary species. Evidence is presented for the first and last of these hypotheses. It is suggested that due to its durable, 3‐dimensional web Cyrtophora species could invade grassland habitats exposed to frequent and heavy wind and rain (Lubin, 1973). Colony formation then evolved in the larger species, C. moluccensis and C. citricola, as a means for exploiting large, open spaces that are rich in flying insects and poor in competing araneids. Under such optimal conditions, low level aggression occurring during prey capture and web construction may be interpreted as instances of reciprocal altruism among related colony members.