Newly described Australian ballista spider builds a spring-loaded snare to catch a single ant species

An international team of researchers have discovered a remarkable new spider species in the rainforest of north Queensland that spins an ingenious and powerful spring-actuated snare to catch a single species of ant – one ant at a time – in what they describe as “the ultimate specialisation”. 

Nicknamed the ballista spider after the ancient Roman weapon that used a spring to launch a bolt or stone, the small nocturnal spider has apparently evolved a unique web mechanism to trap only the highly territorial and aggressive green tree ant Oecophylla smaragdina. 

A detailed description of the spider’s predatory strategy and mechanics is published in the latest issue of the journal Current Biology. The spider – which is yet to be formally named but belongs to the genus Propostira – was first observed by Professor Greg Anderson, a biomedical research scientist who is also a spider taxonomist and photographer. 

Lead researcher Professor Ajay Narendra of Macquarie University and postgraduate student Pranav Joshi then spent 10 days and nights in rainforest near Cooktown in far north Queensland locating the spiders, observing them in detail and capturing their behaviour using high-speed and infrared cameras. 

“It’s very unusual for a spider to feed on ants, because they’re notoriously dangerous, and even more bizarre to find a spider that eats only one particular ant species,” said Professor Narendra. “Ants have a range of chemical defences – including the ability to sting in some species – and they use alarm signals to rapidly recruit hundreds and even thousands of other ants as backup to overcome potential predators.” 

During the day, the ballista spider takes refuge on the underside of a leaf above an area where green tree ants are actively foraging. As night falls, the spider descends 50cm or more to lay an anchor point on a leaf, a branch, or the forest floor, then spends up to four hours engineering a vertical arrangement of 15-60 silk tension lines bundled together in a cone near the ground. 

As a final step, the spider wraps the cone with a thinner type of silk then rapidly retreats upwards. Within seconds, a green tree ant is attracted and reacts aggressively, biting the cone and detaching it from the anchor point. 

With the snare thus sprung, the ant is launched more than 30cm upwards into the spider’s core web at an acceleration of more than 1300 metres per second2. The spider waits for the ant to be fully entangled in its web before approaching and wrapping it with silk. 

“We suspect during the final construction stage the spider adds a pheromone that specifically lures worker ants and induces an aggressive attack, triggering the snare,” said Professor Narendra. “This seems to be the only case where a spider’s web is designed to catch a single prey species, and where the mechanism is triggered by the prey rather than by the predator.” 

Co-senior author Dr Jonas Wolff, who studies biomechanical properties of spider silk, travelled to Australia to observe the spider in the wild and took samples of its silk back to his lab at the University of Greifswald in Germany for detailed physical analysis, including scanning electron microscopy. 

“The ballista spider’s snare is bioengineered to store elastic energy in the silk and rapidly release it, giving it incredible instantaneous power density – greater than any other specialised silk-based biological catapults,” said Professor Narendra. “The ants it preys on have adhesive pads on their feet, so the contraction of the bundle of tension lines has to overcome a force of many times the ant’s body weight to lift it.” 

“The snare mechanism seems to have evolved as a highly specialised way of allowing the spider to ‘pick off’ potentially hazardous prey one at a time and transport them a safe distance away from ant trails and nests.”