Ants are rarely seen foraging in the webs of orb-web spiders, even though creatures trapped there or the resident arachnid would make for an easy meal. Daiqin Li at the National University of Singapore and his colleagues report that the orb-web spider Nephila antipodiana (pictured) deposits an alkaloid compound on its silk strands that ants find repulsive.

Credit: D. LI

Ants of three species were happy to access food via spider silk that had been washed clean, but rarely crossed either untreated silk or silk that had been cleaned and then coated in the alkaloid. Worker ants contacting chemical-laced silk rapidly retreated.

In addition, the web silk threads of small, young spiders that were too thin to support the weight of ants did not contain the chemical. This indicates that it is a deliberate addition by larger spiders, and not a by-product of silk production, the authors write.

Proc. R. Soc. B http://dx.doi.org/10.1098/rspb.2011.2193 (2011)