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Self-assemblages of fire ants are striking behaviors of fire ants to keep its colonies together during the regular flooding of its habitat. Each assemblage has as many as 100,000 individual ants, whose structures are various like rafts, bridges, and towers, and the basic structural unit is the raft formation. The self-assemblages of fire ants allow them to sail upon it for months as they migrate and search for the next lands.

Water repellency of fire ant rafts
The surface of fire ants is hydrophobic, which is a physical property that is seemingly repelled from water. Since their surfaces repel water, a water drop deposited on their head is spherical with the large contact angle since water molecules try to minimize the surface area contacting with the hydrophobic head. Moreover, the fire ant can trap an air bubble inside water due to its water repellency. The hydrophobicity is even enhanced when the fire ants form clusters. The ant clusters submerged in water can capture an air pocket surrounding them. The ant rafts are buoyant and do not sink despite the moderate force by a twig, which arises from the hydrophobicity.

Raft stability
Fire ants grip with each other when forming the self-assemblages by using a combination of mandibles, tarsal claws, and adhesive pads located on the ends of their tarsi. These concrete attachments help the colonies to construct the self-assemblages tightly even under the influence of flood flows. Small-scale rafts composed of less than 10 ants are, however, unstable even under the static water condition despite the strong grips of appendages. The instability of small-scale ant rafts is caused by the mutual repulsion between ant individuals. Ants on the raft perform random walks and expand the raft by accretion of their bodies on the raft edge. Fire ant rafts may also extend pseudopod-like appendages through treadmilling, and morph into streamlined airfoil shapes when in flow. Ant assemblages recognize different fluid flow conditions and adapt their behavior accordingly to preserve the raft’s stability.

Driving forces of self-assemblages
Five forces affect the raft formation assuming that the social attraction is not considered. These force terms include inertia, active propulsion, drag, capillary attraction, and ant-to-ant repulsion. The active propulsion indicates a force term induced by ants who are random walkers and is estimated from their walking data. The drag means the fluid drag acting on a swimming ant that induces the deceleration. Individual ants are drawn together by capillary forces, as if Cheerios cereals floating on the milk are attracted with one another. The short-range elastic repulsion force is generated when two ants collide.

Social interaction
It remains unknown if social attraction forces are legitimate or simply a crutch. The effect of social interactions was not previously considered when modelling the self-assemblages of fire ants for two reasons: i) considering the social attraction increases the number of model parameters to be measured from experiments, and ii) individual insects have limited sensing capabilities and intelligence, which makes it difficult to justify social attraction forces for insect swarms. It was previously addressed that ants would have limited sensory capabilities because the ants walking on the water surface cannot recognize neighboring ants and so ricochet off them.