Pit-building antlions have a two-part strategy for capturing prey. First, they dig conical pits in the sand and bury themselves at the bottom of the pit to wait for a victim.

“If an ant falls into the pit, it tries to run away, but the sand crumbles beneath its feet,” said University of Illinois graduate student Fredrick Larabee, who conducted the study with entomology professor and animal biology department head Andrew Suarez. “This pulls it closer to the center of the pit where the antlion is waiting.”

The antlion sometimes employs its second strategy: It hurls sand at the ant, causing a tiny avalanche that further destabilizes its target. If the ant tumbles to the bottom of the pit and the antlion grabs it with its mandibles, the ant is doomed.

However, one species of ant has physical characteristics that help it survive this deadly trap where other ant species often perish. The mandibles of the trap-jaw ant Odontomachus brunneus can whip shut at speeds over 40 meters per second (144 kilometers per hour), instantaneously maiming a prey insect or enemy ant. They also are used for more routine tasks, such as digging nests or tending to eggs and larvae.

Previous studies have reported that trap-jaw ants sometimes jump with their jaws, “but it was unknown whether this behavior was meant to help them get away from a predator, and it wasn’t clear that it actually improved their odds of surviving an encounter with a predator,” said Larabee.

The new study verifies that the mandibles do in fact aid the ants’ survival by allowing them to eject themselves from a dangerous predicament.

Larabee dropped trap-jaw ants into antlion pits in the lab to see if, and how often, the ants used the jaw-jumping maneuver to escape from an actual predator. The ants usually tried to run out of the pit, and sometimes were successful. If that strategy failed, they sometimes jumped with their jaws.

“The ants were able to jump out of the pits about 15 percent of the time in their encounters with antlions,” Larabee said. “But when we glued their mandibles shut before dropping them in the pits, they couldn’t jump at all. It cut their survival rate in half.”

The study shows how a trait or capability that evolved for one purpose can be adapted for different uses, Larabee said.

“In this case, a tool that is very good for capturing fast or dangerous prey also is good for another function, which is escape,” he said.

The National Geographic Society Committee for Research and Exploration, the National Science Foundation and the American Museum of Natural History supported the research.