The researchers found that only large Golden orb web spiders produce the defensive compound, suggesting that the younger, smaller spiders could rely on their thinner web silk to physically prevent ants being able to climb into their webs. They made the discovery by allowing the Golden orb web spider to spin webs in the lab and then analyzing the compounds in the silk. Once the defensive alkaloid compound was identified, the researchers observed the behavior of ants in its presence.
The Golden orb web spider is typically found in the forests of Australia, Asia, Africa and America.
Associate Professor Daiqin Li, who led the team at the National University of Singapore, said that ants rarely occur on the web of orb web spiders, despite their abundance, so his team set out to discover why.
"We found that large Golden orb web spiders add a defensive alkaloid chemical onto the silk, which stops the ants from walking onto the web when they come into contact with it," said Li.
Professor Mark Elgar from the University of Melbourne's Department of Zoology said the team was impressed by the strength of the ant repellent in the web silk. "The type of chemical deterrent found in the spider silk is known as a pyrrolidine alkaloid, which acts as a predator deterrent in many species of ants, moths and caterpillars.
"The orb spider is potentially vulnerable to attack from groups of ants while sitting in its web waiting for prey, so the chemical defense in web silk may have evolved to not only protect the spider, but to reduce the time and energy that would otherwise be required to chase away invading ants."
Published in the journal Proceedings of the Royal Society B.