A team of biologists and physicists have discovered how antlions optimise the creation of their deadly pits to draw prey into their jaws.

Antlions are small insects with fish-hook sharp jaws that can drain the bodily fluids of their victims within minutes. They build pits lined with fine sand grains that create a slippery surface for prey – typically ants – tumbling them towards the antlion’s jaws.

Now, biologists from the University of Bristol and physicists from Imperial College London have teamed up to work out exactly how they build such deadly pits. Their results are published today in Proceedings of the Royal Society B.

Time-lapse of an antlion digging a pit

Sifting sand

It was known that antlions make pits lined with fine sand grains and that they throw large debris out of the pit while digging. But during their fieldwork, the researchers were amazed at how thorough this is – even sand grains that are only slightly larger are ejected.

They performed an experiment in which they gave antlions a mixture of large and small sand grains and captured and separated all of the grains thrown out of the pit. 

Vastly more large sand grains were thrown out than would have occupied the volume of sand that became the pit.

Professor Nigel Franks from the University of Bristol’s School of Biological Sciences, said: “It is almost like a conjuring trick – many more large grains are ejected than seems possible. 

“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.”

Spiral digging equals efficiency

How this is achieved lies in a physical process called spontaneous stratification, where sand avalanches create alternating layers of small and large grains. This is where Professor Kim Christensen and his PhD student Max Falkenberg, from the Department of Physics at Imperial, came in with their expertise in strange properties of granular mixtures.

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 Professor Kim Christensen

Together they formulated a computational model, mimicking an antlion digging in granular mixtures, to gain insight into the processes and the reasons why they employ spiral digging.

Max explained: “By using simple models for the avalanching of sand grains, we were able to show that the spiral digging technique of antlions maximises the rate at which large grains are filtered out of the pit and minimises the construction time of a pit to its most deadly state, where large avalanches will carry prey into the antlion's lethal jaws.”

By lining their pits with fine slippery sand grains, the antlions make their pits extremely avalanche-prone.

Professor Franks added: “Any prey item that ventures into the pit will ride an avalanche down to the deadly antlion at the bottom of the pit. Such pits are an intriguing example of the ever-present force of natural selection that shapes biology.”

Professor Kim Christensen 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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‘Digging the Optimum Pit: Antlions, Spirals and Spontaneous Stratification’ by Nigel R. Franks, Alan Worley, Max Falkenberg, Ana B. Sendova-Franks and Kim Christensen is published in Proceedings of the Royal Society B.

Based on a press release by the University of Bristol.