Chapter 20: The Social Insects

The Social Insects establishes the rigorous definition of eusociality, which requires cooperative brood care, a reproductive division of labor among sterile and fertile members, and the overlap of multiple generations within a single colony. The text investigates the two primary evolutionary pathways to this state—the parasocial and subsocial routes—highlighting the progression from communal nesting to the development of sophisticated worker castes. Central to the organization of these societies is the concept of castes, which can be anatomical, such as specialized soldiers and queens, or temporal, where individuals transition through different roles like nursing and foraging as they age, a process known as polyethism. Communication is another vital pillar, dominated by an intricate system of chemical pheromones that trigger immediate behavioral releaser effects or long-term physiological primer shifts. The text provides a deep dive into the genetic underpinnings of sociality, particularly the haplodiploid sex-determination system in Hymenoptera, which creates a genetic asymmetry where sisters are more closely related to one another (r = 3/4) than to their own hypothetical offspring (r = 1/2). This provides a robust framework for kin selection, explaining why altruistic labor is evolutionarily advantageous when the sacrifice of an individual increases the fitness of close relatives. The chapter further details the natural history of specific taxa, ranging from the primitive Australian bulldog ants and highly specialized fungus-growing Attini ants to the symbolic waggle dance communication of honeybees and the massive nomadic raids of army ants. It concludes by comparing these Hymenopteran societies with termites, noting that while termites are phylogenetically distinct and lack haplodiploidy, they have achieved comparable social complexity through a mutualistic dependence on wood-digesting microorganisms and anal trophallaxis.