frozen red fire ant raft
June 13, 2014

How Fire Ants Form Living Balls When Faced With Water

Alan McStravick for - Your Universe Online

Solenopsis invicta - the destroyer of picnics and bane of exposed limbs everywhere - is better known by its common name, the red fire ant. Aside from S. invicta's propensity to swarm and attack, it turns out this insect is also one of the more talented engineers in the animal kingdom. Never is this more true than when the colony, threatened by flooding from rain, a sprinkler or a borderline psychopathic child armed only with a cup of water, streams out of the mound en masse rapidly assembling into a raft.

This and other protective structures are formed by each ant gripping onto its nearest neighbor. The mystery behind this behavior is based on the fact that, individually, an ant is denser than water and is therefore at a very real risk of sinking. So just how do they manage to float to safety as a large group? “You can consider them as both a fluid and a solid,” explained David Hu of the Georgia Institute of Technology in a recent statement. Hu's interest in S. invicta stems from the relatively large size of the insect and how that scale presents a more easily observable model of their feats of engineering.

Along with Paul Foster and Nathan Mlot, Hu wanted to investigate one ant structure in particular and how the insects managed to self-assemble into that structure.

To coax the ants into forming their chosen structure of study, an ant ball, the researchers gently swirled 110 ants in a beaker until they formed the living sphere. Once formed, the team immediately flash froze the ants with liquid nitrogen and then coated them with a Super Glue™ vapor to properly preserve the minute points of contact between each ant.

[ Watch the Video: CT Scan For Fire Ant Rafts ]

Once formed and preserved, the researchers enlisted the aid of Angela Lin and her CT scanner. “With the CT scan we can focus on individual ants and see how they are connected to their neighbors,” Hu pointed out. He went on to note that the processing of the images could only be partially automated due to the fact that it is really very difficult to ascertain where one ant ends and the other begins.

This was where the most tedious portion of the study began. For several months, the researchers painstakingly scrutinized each image to arrive at their findings. On average, each ant, according to Hu and his team, participated in 14 contacts. The initial six contacts were made by the individual ant reaching out with all six of its legs and gripping onto its neighbors. The other eight contacts were reciprocated by the neighbors of the individual ant gripping right back. “It turns out that 99 percent of the legs are connected to another ant and there are no free loaders,” stated Hu. The high degree of connectivity was both surprising and impressive.

The next phase of the study relied upon Foster digitally removing all of the individual limb connections from the CT scanner images in order to gain a better understanding of how the insects align their bodies prior to making contact with the other ants. Foster was amazed to see that instead of clustering together in parallel, like grains of rice in a jar, the insects lined up perpendicular to one another. Addressing this finding Hu said, “They have to be alive to do that. It requires some intelligence, and suggests that somehow they sense their relative orientation.”

Each ant of the mound, regardless of size, found they had a particular role to play in forming the ball. The larger ants linked together leaving small gaps that were quickly filled in by the smaller ants. This also led to an overall increase in the total number of contacts.

Another observation made by the team was that the ants’ legs were fully extended and were basically pushing against their neighbor. By doing this, the distance between each ant was increased thus reducing the density of the ball. The air pockets created by this are instrumental in increasing water repellency and buoyancy. This, says Hu, is why their rafts are so effective.

Building on their discovery of self-assembly among the red fire ant community, Hu and team want to continue their work to learn how ants would actively work to reinforce weak points in an ant structure experiencing architectural failure.

The results of this study have recently been published in The Journal of Experimental Biology.

Image 2 (below): By freezing the ants, the Georgia Tech team observed that fire ants construct rafts when placed in water by gripping each other with claw and adhesive pads at a force 400 times their body weight. The result is a viscous and elastic material that is almost like a fluid composed of ant “molecules,” researchers said. Credit: Georgia Institute of Technology

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