After The Deluge

Returning from a wet walk where I saw worms migrating from sodden turf, I wondered where Spring had got to. But I’m fortunate that I live above ground, and that this rain, although it may be steady, could not be described as a deluge. Some of our insect species are not so fortunate and live underground. Think about a poor fire ant (Solenopsis invicta) caught in a deluge. Flash floods are common in the Brazilian rain forest. Caught when they are out for a walk, they will just float away as they have a water repellent coat and are light enough for the surface tension to keep their bodies as well as their heads above water.

But, as we all know, the colony is the critical unit, and they are in a hole in the ground. So what do they do? They build a raft.

The skeptical voices amongst us are already shouting “Out of what?” Easy answer. Out of each other, of course. They hold on to each other leg on leg, leg in mandible, in a tight hydrophobic mass that traps air and floats. Ants pour out of the drowning nest and run across the raft to expand the edges. The process has been mathematically modeled by Mlot et al and is in yesterday’s Proceedings of the National Academy of Sciences (1). Now if we know how many ants there are, we can predict the size and rate of raft building.

The ants don’t care about the equations, and build as long as they can find a free ant. Those trying to abandon ship and make it on their own are firmly grabbed and slotted into the outer periphery of the raft. The trapped air allows the ants below water level to breath and the whole colony can float away until the deluge recedes or they come to higher ground. Together they form a pretty tight ship.

However, researchers being researchers, they had to toss in some detergent, cutting the surface tension in half and making the ants hydrophilic. Bang went their air supply, and not only did they lose the buoyancy of the trapped air, but they can’t breath under water. That was end of their rafting adventure. So far, I have not seen a survivor count.

Fire ants self-assemble into waterproof rafts to survive floods 
PNAS 2011 ; published ahead of print April 25, 2011, doi:10.1073/pnas.1016658108

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