How do trap-jaw ants keep their heads from blowing up?

Jaw-trap ants (Odontomachus brunneus) is a predatory insect with extraordinary jaws. They open their jaws by 180^Hi and lock it in there until they need to close it. The snap, when it happens, takes only microseconds and involves considerable force. In fact, the jaws of these ants are one of the fastest moving appendages in the animal kingdom.

Ants use these jaws to electrocute or kill most of their prey, but can also release their snaps near the ground. This action ultimately propels the ant into the air, either as a kind of “jump” to aid movement, or to get the ant out of a sticky situation.

When not being used to kill prey or jump to safety, the ant’s jaws can be used very carefully to greet other ants or manipulate small pieces of food. Impressed by the way trap-jaw ants use their jaws for delicate maneuvers and fierce punches, a team led by Sheila Patek at Duke University set out to investigate how these jaws work. Interesting results from this study were published in Journal of Experimental Biology.

This species is native to the southern US, Central America and the West Indies. Patek and his colleagues collected specimens from a colony they found in the scrub near Lake Placid, Florida. They dissected several ants and performed detailed measurements and micro-CT scans of the head and jaws. Experts use these measurements to model the ants’ movements and understand what allows them to release their jaws with such force and speed.

Study co-author Chi-Yun Kuo gently secured the ants in front of a high-speed camera and filmed their jaw movements at 300,000 frames per second to capture the lightning-fast maneuvers as the insects brought their jaws together.

Once released, each mandible swings in a perfect arc through 65 . first^Hi before slowing down and stopping. “When we replayed the video in slow motion, their attacks were very accurate,” Patek said.

The researchers found that the ants use two separate spring mechanisms to reach their powerful jaws. First, the muscle stretches the jaw so that there is 180^Hi the corner between them. In doing this, they changed the shape of the sides of the ant’s head, making it slightly shorter (3.2 percent) and narrower (6 percent) in the middle. As the exoskeleton deforms inward, it stores elastic potential energy that can be used when the jaw is released.

Second, the large muscles attach to each jaw via spongy, elastic tendons. Thus torsion is developed by storing elastic energy in two different places on each mandible. When the jaw is released, the head capsule reappears to its normal shape and this pushes the mandible forward, away from the body. At the same time, the elastic tendons pull the internal edges of each jaw toward the body. This causes its jaw to swing in a perfect arc, and reaches a maximum speed of about 120 mph (195 km/h). This movement is equivalent to spinning at 470,000 rpm. The researchers refer to this system as a “double-spring force pair”, because the two springs transmit energy in two different locations at the same time, to each mandible.

Calculating the amount of energy released when insects release their fractured jaws, the team found that the energy stored when the outer head is deformed is enough to propel the lower jaw through 33^Hi of perfect rotation. Energy stored in the spongy tendon that attaches the mandible to the large adductor muscle in the head (comprising 14 percent of the ant’s body mass) energizes the remaining 32.^Hi from the bow.

Researchers are confused as to how this spring system can work without generating excessive friction, which would slow down jaw movement and result in wear and tear on the joints. Using dynamic modeling, they found that the double-spring force pair reduced the need for joint constraints and that the ants had a much stiffer joint structure than they expected. This system reduces friction, which is important if the ant wants to close its jaws repeatedly.

Thus, jaw-trap ants use a mechanism that allows them to coordinate opposing forces that drive the complete rotation of the mandible. Since no pressure is placed on the fragile joint on which the mandible pivots, there is no damage to the ant itself, regardless of how often it closes its jaws.

Patek suspected that other spring-loaded creatures were also using this strategy, and he, Sarah Bergbreiter (Carnegie Mellon University, USA) and Suzanne Cox (Duke University) suggested that the revolutionary design could be embraced by engineers.

“The principles can be incorporated into microrobotics to enhance the multifunctionality, precision, and longevity of ultrafast systems,” they said.

Further details of the study can be found at: https://journals.biologists.com/jeb/article-lookup/doi/10.1242/jeb.244077

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By Alison Bosman, Earth.com Writing Staff

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