Bloody Funny Wasps

Following review of formicid characters a new model for evolution of ant eusociality is presented. This “semisocial ancestry hypothesis” has the following steps: (1) Ants are descended from a solitary wasp which accumulated and collectively reared its brood in acellular nests (without individual brood cells). Metapleural glands delivering antibiotic compounds evolved in adults as a correlated adaptation for nest sanitation. (2) Such wasps gathered to form cooperating family groups of congenerational alate females and their offspring. (3) Female dealation evolved, correlated with pedestrian foraging. Colonies now comprised coeval dealate females rearing alate daughters and sons. Competition generated reproductive and non-reproductive castes in the parental generation, resulting in semisocial colonies, eventually with one dealate mated reproductive queen per nest, and a force of congenerational non-reproductive dealate “workers”. These semisocial formicids progressively acquired petiolar nodes, elbowed antennae, increased longevity and ant-like larvae. (4) Eusociality began when daughter females dealated prematurely and joined the parental-generation worker force. (5) Daughter (eusocial) workers adaptively superceded parental-generation (semisocial) workers and, with the evolution of mechanisms for caste determination, lost mesosomal complexity to constitute eusocial colonies like those of modern ants, with alate/dealate female reproductives and mesosomally-reduced daughter workers. (6) Some of these gave rise to modern ants. (7) Competition between eusocial and semisocial formicids resulted in extinction of the latter, leaving eusocial ants transcendent. These events occurred early in formicid history. The fossil record indicates that eusocial ants referable to several modern subfamilies were present by late Cretaceous times. The semisocial formicids could have included derived species either with morphologically undifferentiated apterous mesosomally-reduced females, or with alate/dealate queens and congenerational apterous workers. The extinct upper Cretaceous Sphecomyrma freyi is possibly the female or worker of one of these. It is proposed that the worker caste in modern ants has evolved more than once, and that eusociality arose several times in the semisocial ancestral group, producing separate lineages of eusocial ants, several of which survived to constitute the modern fauna. The unusual reproductive biology of the aberrant ponerine genus Diacamma is reviewed.