BBC:
A new species of spider which weaves a catapult-like silk trap to snare a single type of ant has been discovered in the remote rainforests of northern Australia.
Researchers believe the nocturnal predator developed the unique hunting method to make meals of aggressive ants which are notoriously dangerous – and unusual – prey for arachnids.
The snare’s “exceptionally high power” flings the ant into a bigger web at “15 times the most extreme g-forces experienced by jet pilots”, said lead researcher Prof Ajay Narendra.
Though it is yet to be formally named, scientists have nicknamed the tiny spider “ballista”, after the ancient weapon used to hurl stones in battle.
“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,” researcher Dr Jonas Wolff said.
Ants have chemical defences, including the ability to sting in some species, and can recruit throngs of other ants rapidly as backup to overcome potential predators, Narendra explained.
Their team, from Australia’s Macquarie University, spent 10 nights in the tropical rainforests of northern Queensland, capturing the spider’s behaviour using high-speed and infrared cameras.
According to their findings, published in the journal Current Biology, the ballista spider resides on trees occupied by the aggressive and territorial green tree ant Oecophylla smaragdina, spending the day in webs hidden beneath the underside of leaves.
After nightfall, it drops down some 50cm to a leaf, a branch or the forest floor and creates an anchor point using a silk line.
It then spends hours creating a cone-shape “scaffold” of dozens of tension lines, around which it finally wraps a thinner type of silk before retreating upwards.
Within moments, scientists found green ants approached the trap and bit it – causing the snare to spring and the prey to be launched into the spider’s web at “extreme” acceleration.
The scientists found that these green ants were the only prey captured by the spider, even when they released other nocturnal ants near the trap. They suspect the spider adds pheromones to the trap to lure and anger the green ants alone.
That is unprecedented, Narendra said.
“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.”
The spider, which belongs to the genus Propostira, was initially observed by biomedical researcher Greg Anderson – also a spider researcher and photographer.
