The Physics That Makes an Antlion’s Sand-trap Nearly Inescapable

The post The Physics That Makes an Antlion’s Sand-trap Nearly Inescapable appeared first on A-Z Animals .

Antlion larvae may not look like much, but they are easily some of the best engineers in the animal kingdom. This insect’s larvae dig incredibly delicate yet effective traps with sand. Now, thanks to multidisciplinary observations, scientists have figured out just how these little creatures design some of nature’s most impressive hunting traps.

Understanding these deceptively simple yet functionally complex sand traps required a meeting of minds. As such, several years ago, biologists from the University of Bristol teamed up with physicists from Imperial College London to figure out exactly how these minuscule larvae use sand to capture a meal. Let’s learn more about antlions and how their toddlers create sand avalanches to lead prey right into their claws.

Antlion Background

The name antlion covers about 2,000 species of insect in the Neuropteran family. While humans usually gain recognition later in life, antlion larvae are the stars of the show due to their tendency to dig sand traps for passing ants and other insects. Americans might recognize them by the name doodlebugs. As for adult antlions, they are less known because they do not dig these fascinating traps and live very short lives. Most of the time, adult antlions—sometimes called antlion lacewings—are confused for dragonflies or damselflies due to their long wings and tendency to fly at dusk.

Antlion larvae, however, look more like beetles than flies. They have reddish-brown bodies and two pincer-like claws at the top of their head. And while there are several thousand tribes of antlions, just three antlion tribes dig traps: Myrmeleontini, Myrmecaelurini, and Nesoleontini. These types of antlion larvae not only dig impressive traps, but they also use their sharp jaws to drain the body fluids of prey in minutes. Scientists have long observed their ability to build sand traps. In 2019, however, they discovered the exact method for the master plan .

Sand Traps

First, antlion larvae find a suitable spot—preferably sand or other fine, earthy material. Then, they get to work. The larvae start by spiraling backward, using their abdomens as plows. This action flicks loosened material from the outer circumference toward the center. Bit by bit, grain by grain, the larvae dig themselves down as the walls of the pit slope upward.

They continue until the walls reach a structural threshold known as the critical angle of repose. This means the walls are at their steepest possible angle before collapsing. The larvae, safe at the bottom of the pit, hide under the material with only their pincer claws projected out . Sooner or later, prey falls into the trap, riding the avalanche of sand to the bottom, right into the claws of the antlion larvae.

It’s natural deception at its finest. However, scientists weren’t exactly sure how these small insects could build the traps without them collapsing constantly. In 2019, scientists from the University of Bristol and the Imperial College of London teamed up to take a closer look.

Running Tests

The research team experimented. They gave antlion larvae a mix of large and small sand grains, then captured and separated every grain the larvae threw out of their burgeoning pits. Quickly, the researchers realized that considerably more large grains were thrown out “than would have occupied the volume of sand that became the pit.” It seemed like a conjuring trick; far more large grains were ejected than seemed possible. As Professor Nigel Franks from the University of Bristol explained in one article , “One answer is that the antlion is ‘interrogating’ much more sand to find and eject large grains than just the volume of sand that has to be removed to form the final pit.”

At this point, physics entered the equation. Professor Kim Christensen and his PhD student, Max Falkenberg from the Physics Department at Imperial College of London, offered their expertise. They realized this ‘conjuring trick’ was actually a process called spontaneous stratification. This occurs when sand avalanches create alternating layers of small and large sand grains.

To confirm their observations, they created a computational model that mimicked an antlion’s digging process. As Falkenberg explained , “By using simple models for the avalanching of sand grains, we were able to show that the spiral digging technique of antlions maximizes the rate at which large grains are filtered out of the pit and minimizes the construction time of a pit to its most deadly state, where large avalanches will carry prey into the antlion’s lethal jaws.”

Fibonacci Ambush

Perhaps it should come as no surprise that antlions dig in spirals to achieve critical angles of repose. Spirals, after all, are found throughout nature, and are some of the most mathematically efficient shapes possible. It is a stroke of genius, however, for antlions to line their pits with fine, slippery grains. This makes the walls prone to avalanching with the slightest pressure—sending prey right into their clutches.

As Professor Franks explained , any prey creature that goes into one of these pits is almost guaranteed to ride to the bottom. He said, “Such pits are an intriguing example of the ever-present force of natural selection that shapes biology.”

As for the team’s insights, Professor Kim Christensen credited the collaboration between two different scientific disciplines. He said , “Science would never have revealed the secrets of why antlions employ spiral digging were it not for a genuine interdisciplinary work between biologists and physicists, a key focus of the Centre for Complexity Science here at Imperial.”

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