A newly discovered spider in the rainforests of northern Queensland has evolved an amazing hunting technique. Researchers say the species has the “ultimate special ability” to use powerful spring-loaded silk traps to capture a single type of prey, capturing ants one at a time.
These small nocturnal arachnids are called ballista spiders, after ancient Roman siege engines that used built-up tension to fire bolts and stones. Scientists say the spider appears to have developed a unique web system designed specifically to trap aggressive green ants. Oecophila smaragdina.
The discovery and detailed analysis of spider hunting behavior was published in the magazine current biology. This species has not yet been officially named, but it belongs to the genus Propostira. The spider was first discovered by biomedical researcher, spider taxonomist and photographer Professor Greg Anderson.
Macquarie University researchers led by Professor Ajay Narendra and graduate student Pranav Joshi spent 10 days and nights searching for spiders in the rainforest near Cooktown in far north Queensland. Using high-speed and infrared cameras, they meticulously documented the animals and recorded their amazing behavior.
a spider that specializes in hunting dangerous ants
Professor Narendra said: “It is very unusual for spiders to eat ants. Ants are notoriously dangerous and it is even stranger to find a spider that only eats one type of ant.”
“Ants are equipped with a variety of chemical defenses, including the ability to sting some species, and they use alarm signals to quickly recruit hundreds or even thousands of other ants in reserve to overcome potential predators.”
During the day, ballista spiders hide under leaves above areas where green ants are actively foraging. When it gets dark, they descend more than 50 cm and create anchor points on leaves, branches, and the forest floor.
The spider then spends up to four hours constructing an elaborate vertical trap. The structure consists of 15 to 60 taut silk strands tied together and arranged in a conical shape near the ground.
How the Ballista Spider Silk Catapult Works
To complete the trap, the spider wraps the cone in a thin kind of thread and immediately retreats upwards. After a while, a green ant approaches you. The ants react aggressively and bite the cone, accidentally separating it from the anchor point.
That action triggers a trap.
When released, the spring-loaded trap launches the ant more than a foot upwards into the spider’s main web. Ants experience accelerations of more than 1300 meters squared per second before becoming entangled in a nest. Only when the prey is completely restrained, the spider approaches it and wraps it in a thread.
“We think that during the final stages of construction, this spider added pheromones that specifically attract worker ants, triggering aggressive attacks and triggering traps,” Professor Narendra said.
“This seems to be the only case where a spider’s web is designed to capture a single prey species, and the mechanism is triggered by the prey rather than the predator.”
A biological catapult powered by silk
Co-senior author Dr. Jonas Wolff, an expert on the biomechanical properties of spider silk, traveled to Australia to observe spider silk in its natural habitat. He then took the silk samples back to the University of Greifswald in Germany, where they were examined using advanced techniques such as scanning electron microscopy.
“Ballista Spider’s snare is bioengineered to store elastic energy in silk and release it rapidly, resulting in incredible instantaneous power density, which is greater than other specialized silk-based biological catapults,” Professor Narendra said.
“Because the predatory ant’s legs have sticky pads, the contraction of the bundle of tension lines must overcome forces many times the ant’s weight to lift it.”
Researchers believe this mechanism evolved as a highly specialized solution for safely hunting dangerous prey.
“The trapping mechanism appears to have evolved as a highly specialized way for spiders to ‘pick’ potentially dangerous prey one at a time and transport them to a safe distance from ant trails or nests.”
