Spiders in Singapore have been discovered to be adding chemicals to their silk to deter ant attacks.

Spider webs should make fertile hunting grounds for predatory ants, consisting as they often do of a selection of helpfully immobilized insects and a very edible (from an ant perspective) arachnid. But Daiqin Li, of the National University of Singapore, notes that ants are not often seen foraging in webs.

Li and his team set out to look at what the reason might be. They found that the orb-web spider Nephila antipodiana (pictured here dining) deposits an alkaloid compound on its silk strands that the ants find repulsive.

“This is the first report of the production of chemical components on spider web silks that act as a deterrent against natural enemies,” the team reports in Proceedings of the Royal Society B, “although similar properties have been widely suspected but with mixed empirical support.”

Their empirical support consists of setting up threads from N. antipodiana spiders in the lab and either removing the chemical from the silk, adding it to the silk or leaving the threads as is. Freshly killed ant treats were put near an ant colony, which could reach them only via the threads. Although pharaoh ants (Monomorium pharaonis) were happy to cross the silk without the chemical, they rarely crossed either the natural silk or the added-alkaloid silk. Worker ants contacting chemical-laced silk “rapidly retreated” the team reports.

Follow-up studies confirmed that two other species of ant were also put off by the chemical. Crucially, Li’s research established that the webs of small, young spiders, which were too fragile to support the weight of ants, did not contain the chemical deterrent. This indicates that it is a deliberate addition made by larger spiders, and not some by-product of silk production, they write.

Image: Li Daiqin