'Social' insects and arachnids exhibit forms of complex behaviour that involve cooperation in building a nest, defending against attackers or rearing offspring. This book is a comprehensive, up-to-date guide to sociality and its evolution in a wide range of taxa. In it, leading researchers review the extent of sociality in different insect and arachnid groups, analyse the genetic, ecological and demographic causes of sociality from a comparative perspective and suggest ways in which the field can be moved on. It contains fascinating accounts of the social lives of many different insects and arachnids, as well as tests of current theories of the evolution of social behaviour. The Evolution of Social Behaviour in Insects and Arachnids provides essential reading and insight for students and researchers interested in social behaviour, behavioural ecology, entomology and arachnology.