Researchers have discovered a spider in the rainforests of northern Queensland that employs a unique spring-loaded silk trap to capture a single type of ant, one at a time. The species, nicknamed the ballista spider after the ancient Roman siege weapon, uses stored tension in its web to launch prey upward into a deadly snare.
Discovery and Naming
The small nocturnal arachnid, yet to be formally named, belongs to the genus Propostira. It was first spotted by Professor Greg Anderson, a biomedical researcher and spider taxonomist. The findings were published in the journal Current Biology.
Hunting Technique
The spider specializes in hunting the aggressive green tree ant (Oecophylla smaragdina), a notoriously dangerous prey. “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 Ajay Narendra of Macquarie University, who led the study with postgraduate student Pranav Joshi.
During daylight, the spider hides beneath a leaf above ant foraging areas. After dark, it descends over 50 centimeters to create an anchor point on a leaf, branch, or forest floor. It then spends up to four hours constructing a vertical trap made of 15–60 tensioned silk strands bundled into a cone shape near the ground. The spider wraps the cone with thinner silk and retreats upward.
Trigger Mechanism
When a green tree ant approaches and bites the cone, it detaches from the anchor point, releasing the spring-loaded snare. The ant is launched more than 30 centimeters upward into the main web, experiencing acceleration exceeding 1,300 meters per second squared. The spider only approaches after the prey is fully restrained.
Professor Narendra noted, “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. 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.”
Biomechanical Analysis
Co-senior author Dr. Jonas Wolff, an expert in spider silk biomechanics, traveled to Australia to observe the species and later analyzed silk samples at the University of Greifswald in Germany using 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 specialized 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.”
Evolutionary Significance
Researchers believe the mechanism evolved as a highly specialized solution for safely hunting dangerous prey. “The snare mechanism seems to have evolved as a highly specialized 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,” added Narendra.
