Ever wonder how a colony of ants could survive a flood? Apparently the Brazilian fire ant, Solenopsis invicta, can self-assemble into hydrophobic rafts of thousands of individuals. These rafts can remain afloat for days at time without drowning the individual ants.

It turns out that individual ants are hydrophobic, or water repelling. However, the ants are slightly denser that water so they must rely on surface tension to remain above the water. Surface tension only works for relatively small objects, so how can a raft of thousands of individual ants remain afloat?

The ants can form a small pocket of air around their bodies called a plastron. In large groups, such as in the raft, the plastron can grow even larger, decreasing the density of the raft up to 75 percent. The plastron keeps the water out of the raft and also provides air for the ants below the waterline to breath.

To see how the raft was held together, the researchers froze ant rafts with liquid nitrogen and imaged them with an electron microscope. The ants held onto each other by hooking their feet together and by gripping each others' legs with their mandibles. This allows for strong but non-permanent attachments.

When the ant raft is perturbed or submerged, all the ants contract their bodies in unison. This contraction shrinks the raft and strengthens it in the process. The contracted raft also holds the plastron tighter, allowing the raft to be submerged several centimeters without significant loss of air.

These ant rafts are self-assembling as well as self-healing. A mass of ants dropped into water will form a raft within a few minutes. Removing an individual ant from the raft and the rest of the ants will rearrange themselves to fill the gap. The researchers note that these behaviors are some of the characteristics of a living super-organism.



PNAS, 2011. DOI: 10.1073/pnas.1016658108

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