Fire ants link up into waterproof rafts

Many insects, such as the aptly-named water-striders, are able to exploit surface tension and literally walk on water. However, the majority are only moderately hydrophobic, and because they are denser than water, have to struggle all the harder to remain afloat the larger they are.

Fire ants live in places which are frequently inundated by water. Individual ants placed in water can float, but not very well. How then do they cope with flooding? It turns out that they assemble themselves into ant 'rafts', cooperating to improve their buoyancy and waterproofing.

Insect cuticle itself is already water-repellent to some extent, but alone it is not enough to explain how a clump of ants, gripping onto each other mandible-to-leg, is more buoyant per unit mass than an individual ant. The answer lies in their ability to trap a layer of air (called a 'plastron') around their bodies. By clumping together, they increase the size of these plastrons, and can entrap larger air bubbles within the clump. These lower the effective density of the ant mass, and also allow them to breathe (insects breathe through pores in their cuticle) even when submerged.

Quotable quotes from the paper:

"Ants were scooped with spoons into 100-mL beakers rimmed with talc powder and weighed to count their numbers. Using the natural adhesion of the ants, a few swirls of the beaker was sufficient to roll the ants into balls."
"Not surprisingly, ant spheres that are placed on solid surfaces quickly disintegrate as the ants flee in all directions."
"By harnessing two live ants with an elastic band, we found that the maximum tensile force between them is F = 620 ± 100 dyn (N = 11), or more than 400 times body weight."

Reference:
Mlot, Tovey, & Hu. "Fire ants self-assemble into waterproof rafts to survive floods." Proceedings of the National Academy of Sciences, USA (10 May 2011) vol. 108 no. 19 pp. 7669-7673.