12. Insect societies
The study of insect social behaviors is a popular entomological topic and there is a voluminous literature, ranging from the popular to the highly theoretical. The proliferation of some insects, notably the ants and termites, is attributed to the major change from a solitary lifestyle to a social one.
Social insects are ecologically successful and have important effects on human life. Leaf-cutter ants (Atta spp.) are the major herbivores in the Neotropics, and in south-western US deserts, harvester ants take as many seeds as do mammals. Ecologically dominant “tramp” ants can threaten our agriculture, outdoor behavior, and biodiversity (Box 1.2). Termites turn over at least as much soil as do earthworms in many tropical regions. The numerical dominance of social insects can be astonishing, with a Japanese supercolony of Formica yessensis estimated at 306 million workers and over 1 million queens dispersed over 2.7 km2 amongst 45,000 interconnected nests. In West African savanna, densities of up to 20 million resident ants per hectare have been estimated, and single nomadic colonies of driver ants (Dorylus sp.) may attain 20 million workers. Estimates of the value of honey bees in commercial honey production, as well as in pollination of agricultural and horticultural crops, run into many billions of dollars per annum in the USA alone. Social insects clearly affect our lives.
A broad definition of social behavior could include all insects that interact in any way with other members of their species. However, entomologists limit sociality to a more restricted range of co-operative behaviors. Amongst the social insects, we can recognize eusocial (“true social”) insects, which co-operate in reproduction and have division of reproductive effort, and subsocial (“below social”) insects, which have less strongly developed social habits, falling short of extensive co-operation and reproductive partitioning. Solitary insects exhibit no social behaviors.
Eusociality is defined by three traits:
- Division of labor, with a caste system involving sterile or non-reproductive individuals assisting those that reproduce.
- Co-operation among colony members in tending the young.
- Overlap of generations capable of contributing to colony functioning.
Eusociality is restricted to all ants and termites and some bees and wasps, such as the vespine paper wasps depicted in the vignette of this chapter. Subsociality is a more widespread phenomenon, known to have arisen independently in 13 orders of insects, including some cockroaches, embiids, thysanopterans, hemipterans, beetles, and hymenopterans. As insect lifestyles become better known, forms of subsociality may be found in yet more orders. The term “presociality” often is used for social behaviors that do not fulfill the strict definition of eusociality. However, the implication that presociality is an evolutionary precursor to eusociality is not always correct and the term is best avoided.
In this chapter we discuss subsociality prior to detailed treatment of eusociality in bees, wasps, ants, and termites. We conclude with some ideas concerning the origins and success of eusociality.