When water levels rise, red fire ants (Solenopsis invicta) stream from their nests and rapidly grip onto their nearest neighbors in order to form rafts to carry them to safety. Each individual ant is denser than water and is in danger of sinking. However, the ants somehow manage to stay afloat, and they don’t just draw the line at constructing rafts — they routinely form bivouacs, assemble towers, and even coalesce into droplets when swished in a cup.
“You can consider them as both a fluid and a solid,” said David Hu from the Georgia Institute of Technology, who teamed up with Paul Foster and Nathan Mlot to investigate how balls of living fire ants self-assemble.
The researchers discovered that fire ants form complex networks of connections between their bodies and limbs, align their bodies perpendicularly, and push against each other with their limbs to reduce density and increase buoyancy, which they have described in a recent article in the Journal of Experimental Biology.
Gently swirling 110 ants in a beaker to form a sphere, the team then swiftly froze the structure in liquid nitrogen and coated it in Super Glue™ vapor to preserve the minute contacts within so their images could be captured using a a CT scanner.
“With the CT scan we can focus on individual ants and see how they are connected to their neighbors,” Hu explained.
After months of painstaking scrutiny, Foster and Hu discovered that on average, each ant participated in 14 contacts — reaching out with all six legs to grip neighbors, and receiving eight contacts back to its body — although large ants participated in as many as 20 contacts and the smallest ants participating in only eight.
“It turns out that 99% of the legs are connected to another ant, and there are no free-loaders,” said Hu, who admits that he was impressed by the high degree of connectivity.
Next, Foster digitally removed all of the limb connections so that he could take a closer look at the ways that the ants’ bodies packed together, and he was amazed to see that instead of clustering together in parallel, like grains of rice in a jar, the ants had actively oriented their bodies perpendicular to each other.
“They have to be alive to do that,” said Hu. “It requires some intelligence, and suggests that somehow they sense their relative orientation.”
The duo also analyzed how closely the ants’ bodies packed together and realized that the smaller ants were packing in to fill the gaps between the larger ants to increase the number of contacts. They also noticed that the ants were actively pushing on each other, using their legs like tiny jacks to increase the distance between neighbors and to reduce the density of the ball. Hu explained that by introducing air pockets between their bodies, the ants increase their water repellency and buoyancy, which is why their rafts are so effective.
Finally, Hu and Foster took a closer look at the contacts made by individual ants with a scanning electron microscope and saw that the insects rarely used their mandibles to grip on to other ants. Instead they mainly used their legs, holding on with hooks on their feet and the sticky pads that allow them to walk on vertical surfaces.
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