For red fire ants (Solenopsis invicta), rain gently drumming on the ground is the trigger for a mass exodus. Streaming from their nest as the water levels rise, the ants rapidly assemble and grip onto their nearest neighbours, forming a raft to carry them to safety. What is more miraculous is that each individual ant is denser than water and in danger of sinking if submerged. However, the ants 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’, explains David Hu from the Georgia Institute of Technology, USA, who is most interested in the ants because they are large enough for him to begin to find out how they interact to pull off these remarkable engineering feats. Hu teamed up with Paul Foster and Nathan Mlot to investigate how balls of living fire ants self-assemble (p. 2089).
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™ vapour to preserve the minute contacts within, ready for Angela Lin to visualise the structures in a CT scanner. ‘With the CT scan we can focus on individual ants and see how they are connected to their neighbours’, explains Hu, who adds that processing the images could only be partially automated because it is hard to tell where one ant ends and another begins.
However, 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 neighbours 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’, says 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,’ says Hu, adding, ‘It requires some intelligence, and suggests that somehow they sense their relative orientation.’ The duo also analysed 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 neighbours and reduce the density of the ball. Hu explains 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.
So, having discovered how fire ants self-assemble to form light but stable structures, Hu is keen to know how they react to reinforce weak points in structures where ant architecture could fail.