The ant stalks the forest floor, its antennas alert and its mouth agape. Its sideways jaws are edged with menacing teeth and held so far open they extend back behind the ant's head. There's a twitch in the leaf litter — a tasty critter called a springtail emerges. In an instant, one seven-hundredth the time it takes to blink an eye, the ant snaps its jaws shut around the unsuspecting creature.
Dinner is served.
The mechanics of this formidable hunting strategy have long eluded scientists. These predatory ants, from the genus Myrmoteras, live in small colonies deep in the forests of Southeast Asia and are almost impossible to discern amid the detritus that carpets the ground.
But thanks to a lucky discovery and some high-speed cameras, evolutionary biologist Fred Larabee was finally able to watch this type of trap-jaw ant in action. His study, published Wednesday in the Journal of Experimental Biology, is the first to explain the spring-loaded mechanism that makes these ants' jaws snap.
“They're just fascinating to me,” said Larabee, whose Twitter handle is @bugbiter and whose bio proclaims his interest in “insect mouthparts.”
Larabee, now a postdoctoral fellow at the National Museum of Natural History and George Washington University, wasn't always an ant-jaw enthusiast. After graduating from college, he sought out work as a biochemist. But then he read a journal article by University of Illinois Urbana-Champaign scientist Andy Suarez, who had observed a different group of ants snapping their jaws against the ground as a way to swing themselves into the air and escape predators.
“They were describing this crazy defensive behavior … and I couldn’t believe that it was the sort of thing that people got paid to do,” Larabee said. “It seemed like such a cool thing to study.”
So Larabee became a student at UIUC, and went to work in Suarez's lab. He became a specialist in species with fast-acting “trap jaws” — of which there are many. This ability has evolved at least four times in unrelated groups of ants.
“It's a really good example of convergent evolution,” Larabee said, “where different body parts are being used to do the same function in these different groups.”
In those other groups, the trap-jaw works like a catapult or a mousetrap. The insects hold their mouths open wide, transferring energy to a spring mechanism inside their skulls. When they're ready to strike, a trigger releases the spring, and the jaws snap shut.
For Myrmoteras, “the big questions we wanted to know were how fast the jaws were and if the jaws worked the same way,” Larabee said.
But he had to get his hands on the creatures. Live ones. Myrmoteras, like many hard-to-spot insects, is known only because of a technique called leaf litter sampling, in which scientists sift cupfuls of forest floor debris into a pan of alcohol. It's great for finding new species, “but they end up being dead, which isn’t very useful for trying to measure their speed or behavior,” Larabee said.
Then Suarez — now Larabee's PhD adviser — called to say he'd gotten hold of some Myrmoteras on a collecting trip in Borneo. Larabee was ready to get to work.
First he recorded them with a high-speed camera that captured 50,000 frames per second. The imagery revealed that Myrmoteras' jaws close at 50 mph. But Myrmoteras is still only half as fast as its distant cousin Odontomachus — the ant Suarez observed using its trap jaw to bounce off the ground. The bites of those ants rank among the fastest animal movements in the world.
That said, a Myrmoteras's entire strike takes just about half a millisecond — a 10th of the time it takes a honeybee to flap its wings.
To figure out how it all happens, Larabee next stuck his ants in a micro-CT scanner, which let him probe the insects' internal organs with X-rays. Just like other trap-jaw groups, he found that Myrmoteras relies on a spring to power its predatory strike. But the mechanics of the Myrmoteras jaw are distinct. As the ant hunts with its massive mouth gaping — “almost like walking around with a big bear trap on the front of your face,” Larabee said — a lobe at the back of its head begins to compress. This acts like a spring, loading up potential energy. When it's time to bite, a different group of fast-contracting muscles releases a trigger that snaps the jaw shut.
“They don't use the same system” as other trap-jaw ants, noted Larabee, yet they achieve a similar effect. “It means that these spring-loaded jaws are very useful; otherwise, they wouldn’t evolve so many times.”
The utility of a spring-loaded jaw becomes apparent when you look at Myrmoteras's favorite prey — springtails. As their name suggests, these flea-like arthropods have also mastered the physics of the spring. They use an appendage called the furcula to launch themselves into the air when threatened.
It takes a spring to catch a springtail, apparently.
“It really gives a window into how evolution works,” Larabee said.