A new species of spider, described in the journal Current Biology, was discovered by a team of researchers in the rainforest of north Queensland, Australia.
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Nicknamed the ‘ballista’ spider in reference to the ancient Romana artillery machine resembling a giant crossbow, this small nocturnal arachnid favours the territorial and aggressive green tree ant (Oecophylla smaragdina) as its 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,” says lead researcher Ajay Narendra, from Sydney’s Macquarie University.
“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.”
The spider hasn’t been formally described yet but it belongs to the genus Propostira. During the day, it stays on the underside of a leaf above a green tree ant foraging area.
When the night falls, the spider gets to work – it descends on a silk line, lays down an anchor point and returns to the core web by laying a tension line. It repeats that process until a cone-shaped arrangement forms. Finally, the spider wraps the cone with a thinner type of silk and retreats. And then it waits.
It doesn’t have to wait long – within seconds, a green tree ant is attracted to the trap. It probes the cone with its antennae and immediately becomes aggressive and bites into it. That destabilises the trap, which detaches from the anchor point and launches the ant more than 30cm upwards into the spider’s main web at an acceleration of over 1300 m/s2.
“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,” explains 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.”
After this successful catch, the spider doesn’t approach right away – it waits until the ant is fully entangled in its web, and only then gets closer to wrap it with silk.
In one out of 12 observed instances, the ant triggered the trap but was not hauled up – and without its mass, the snare accelerated at an impressive rate of over 4,700 m/s2.
“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.
“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,” concludes Narendra.
Read the full findings here.
Top image credit: Pranav Joshi
