Few insects in the cool temperate northern hemisphere are as conspicuous as red wood ants. Most red wood ants build large and enduring nest mounds reaching more than 2 m in height, penetrating considerably further beneath the ground surface and thatched on top with a layer of plant material. These mounds are marvels of engineering with a complex array of tunnels and chambers in a carefully controlled environment. Nests may be interlinked, forming, in extreme cases, vast colonies of up to 400 million individual ants covering an area of more than 2.5 km2. Their sheer abundance and collective biomass in temperate forests can rival any mammalian predator.

Red wood ants properly belong to the subgenus Formica sensu stricto or F. rufa group, and can also be commonly referred to as true wood ants, thatch ants or mound ants (hereafter ‘wood ants’). These names derive from the habitat, nesting habit and colour of the adults, which are bicoloured red and brownish-black. Contained within this group of ants are several morphologically and ecologically similar species distributed throughout the Holarctic.

More impressive perhaps than even their numerical dominance is their ecological significance in driving ecosystem processes and function both above and below ground. With a role in predator–prey dynamics, nutrient cycling, seed dispersion, habitat provision and modification, and plant and tree growth, wood ants can truly be described as keystone species. As a result of their ecological importance, wood ants are protected by law in some countries, yet their populations remain threatened from a variety of sources, not least, loss of suitable habitat. This chapter describes species that are currently considered members of the subgenus Formica sensu stricto; their evolution, identification, habitat and distribution. The concept of red wood ants as social insects is introduced. The rest of the book will cover reproductive biology and social systems (Chapter 2), population genetics (Chapter 3) and ecology (Chapter 4), colony and species recognition (Chapter 5), interspecific competition (Chapter 6), foraging and interactions with other organisms (Chapters 7 and 8), nutrient cycling (Chapter 9), similarities and differences between North American and Eurasian wood ants (Chapter 10), monitoring and conservation (Chapters 11 and 12) and future research questions (Chapter 13).