In insect societies, a number of very striking collective structures are formed by individuals linking themselves to one another. One such example is an army ant bivouac. These structures are termed self-assemblages and are part of a more general and important aspect of insect societies - intermediate-level parts - in which functional group-level adaptive structures are formed. These parts are, in a sense, the tissues and organs of complex insect societies. Here we review the natural history of self-assemblages in insect societies. We find that at least 18 different types of structure exist: bivouacs, bridges, curtains, droplets, escape droplets, festoons, fills, flanges, ladders, ovens, plugs, pulling chains, queen clusters, rafts, swarms, thermoregulatory clusters, tunnels, and walls. These self-assemblages are found in a variety of species of ants, bees, and wasps, but (as far as we are aware) not in termites. The function of these self-assemblages can be grouped under five broad categories which are not mutually exclusive: 1) defence, 2) pulling structures, 3) thermoregulation, 4) colony survival under inclement conditions, and 5) ease of passage when crossing an obstacle. The paucity of our knowledge concerning the factors that favour self-assemblage formation and the likely proximate mechanisms are highlighted.
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