Intermediate-level parts in insect societies: adaptive structures that ants build away from the nest

Insect societies function at various organisational levels. Most research has focused on one or other organisational extreme. At one extreme, it is the adaptive behaviours at the individual level, the behaviour of workers, which is of interest. At the other extreme, colony-level adaptive behaviour and swarm intelligence is the focus. However, between these two extremes, numerous functional adaptive units, or "parts," exist. These intermediate-level parts include the behavioural properties of "groups" or "teams" in which the functionality only emerges at the group-level and not within the individuals themselves, and also the structural properties of "self-assemblages" in which individuals link themselves together to form an adaptive configuration, such as a living bridge. We review another type of intermediate-level part in insect societies: these are the physical structures that ants build away from the nest. The structures, that are larger than an individual worker but smaller than the colony (hence intermediate), include cleared traits, walled trenches, arcades, tunnels, outstations, shelters, protective pens, shelters over nectaries, food coverings on foraging trails, elevated corridors, and bridges. They are found in a diverse range of species, and are constructed using a variety of materials. We detail the structures built by ants focussing chiefly on the adaptive benefits these structures may confer to the colony.