Pictures: Fire Ant Swarms Form Living Life Rafts

Team Effort

Photograph courtesy David Hu and Nathan J. Mlot

A pile of 500 fire ants floats on water in a picture taken during the lab experiment.

When a cluster of ants is placed on the water, ants near the top try to leave the pile, Mlot said. But when an escaping ant reaches the edge and realizes that terra firma is nowhere to be found, "usually the ant will turn around and head back toward the center.

"By the time she realizes there's an edge, there's already another ant clambering on top of her, forcing that first ant to become part of the bottom layer." In this way, ants that might have tried to survive solo are instead trapped into becoming part of the raft.

(Also see pictures: "'Zombie' Ants Controlled, Decapitated by Flies.")

Published April 25, 2011

Unsinkable

Photograph courtesy David Hu and Nathan J. Mlot

The finished ant raft is quite buoyant, as the researchers found out after attempting to sink a raft with a twig, pictured above.

During such "perturbations," the ants contract their muscles, which makes the raft temporarily less buoyant but traps air better, preventing drowning. 

Published April 25, 2011

Ant Air Bubble

Photograph courtesy David Hu and Nathan J. Mlot

When the researchers put ants in a dry beaker and gave the container a swirl, the ants rolled into a ball—much like a rolling snowball, Mlot said. It was easy to grasp the ball with a pair of tweezers, as shown above, and submerge the ants (pictured), he added.

A shimmering layer within the underwater ball shows the edges of an air bubble trapped by the "ant sphere." If the ball had been placed on top of the water, the ants would have instead formed a raft. (Related: "Water Spider Spins Its Own 'Scuba Tank.'")

Published April 25, 2011

Natural Buoyancy

Photograph courtesy David Hu and Nathan J. Mlot

Even a single ant, such as this one being held underwater, is buoyant in clean water, thanks to rough, waxy hairs that trap air around the insect's body. Soap and other surfactants can reduce the surface tension of water, which in turn reduces an ant's buoyancy. (Also see "Hairy Legs Help Bugs Walk on Water.")

To study how the ant's exoskeleton traps air, the team needed to weigh the ant down. "The only way we could keep an ant underwater was by tying an elastic band around its body and attaching a weight to it," Mlot said.

Published April 25, 2011

Walking on Water

Photograph courtesy David Hu and Nathan J. Mlot

A lone ant steps onto the water's surface during the lab experiment.

Despite being denser than water, a single ant can walk on water, thanks to surface tension and hydrophobic (water-repelling) feet. But surface tension is too weak to support larger objects, which is why it's been a mystery how ant rafts could stay afloat.

The answer lies in numbers: When linked together in a raft, the ants' collective water repellency was actually 30 percent higher than that of an individual ant, the researchers found. (Related: "How Snails Walk on Water Is a Small Miracle.")

Published April 25, 2011

Ant Lifeline

Photograph courtesy David Hu and Nathan J. Mlot

The fire ant is the only ant species that's been observed forming rafts. But it's been well known that several types of ant can form clusters to create structures similar to towers or bridges. (Related: "Surprising Ant 'Mixing Bowl' Found in Manhattan.")

The Georgia Tech team found that fire ant clusters act like fluids with predictable physical properties: A cluster is a fifth as dense as water but has ten times the surface tension and is ten million times more viscous.

In fact, ant clusters act less like ants and more like putty when handled, Mlot said. "You could pick up that ball and it would have the same texture as soft putty. You could give it a squeeze. You could toss it in the air and the ants would stay together."

Published April 25, 2011

Jaws of Life

Image courtesy David Hu and Nathan J. Mlot

After watching the ants form rafts, the team froze the rafts in liquid nitrogen to study their structures. Ants had linked up using either a "hand to hand" grip, where each ant would grab another's leg, or a mandible-to-leg grip, as seen above in a microscope image.

Either way, the formation of ant rafts is a delicate business: The maximum strength two ants can grasp each other without causing harm is about 400 times their body weight, which is "significantly weaker than ant attachment to other complex surfaces," the paper authors wrote.

Published April 25, 2011

Spot of Tea

Photograph courtesy David Hu and Nathan J. Mlot

When fire ants are gathered into a group, they act like a fluid, as depicted above in a scene, arranged by the researchers, that could be called either whimsical or the stuff of nightmares.

Modeling a group of fire ants as a fluid, with each ant representing a molecule, makes describing an ant raft much like describing a drop of oil spreading on the surface of water, the study authors say.

Of course, ant "droplets" do not follow totally predictable rules, with individual ants moving randomly within the raft, unlike individual oil molecules. (Related: "Gulf Oil Spill Anniversary: News and Pictures.")

Published April 25, 2011