Ants cross a strand of spider silk without 2-pyrrolidinone (top), but avoid the strand with the repellent (bottom). Adapted from Figure 1 of Zhang et al 2011.

One measure of a predator’s ecological significance is the abundance of strategies prey adopt to avoid being eaten. And how ecologically significant are ants?

They are enough of a problem to web-building spiders that the arachnids impregnate their webs with ant-deterring 2-pyrrolidinone:

…ants are rarely reported foraging on the webs of orb-weaving spiders, despite the formidable capacity of ants to subdue prey and repel enemies, the diversity and abundance of orb-web spiders, and the nutritional value of the web and resident spider. We explain this paradox by reporting a novel property of the silk produced by the orb-web spider Nephila antipodiana (Walckenaer). These spiders deposit on the silk a pyrrolidine alkaloid (2-pyrrolidinone) that provides protection from ant invasion. Furthermore, the ontogenetic change in the production of 2-pyrrolidinone suggests that this compound represents an adaptive response to the threat of natural enemies, rather than a simple by-product of silk synthesis: while 2-pyrrolidinone occurs on the silk threads produced by adult and large juvenile spiders, it is absent on threads produced by small juvenile spiders, whose threads are sufficiently thin to be inaccessible to ants.

These results were reported today in a paper by Shichang Zhang in Proceedings of the Royal Society B. Zhang et al assayed three species of ants against an array of silk strands with and without the alkaloid, finding they would more frequently traverse the 2-pyrrolidinone-free strand to reach a food source. It’s good, basic experimental chemical ecology.

I do have one quibble with the study, though. The authors wanted to test the efficiency of 2-pyrrolidinone against ants generally, so they employed several ant species. But their subjects (Pheidole angulicollis, Monomorium pharaonis, and Monomorium indet. sp.) are all relatively similar ants in the subfamily Myrmicinae, and ones I’m not sure represent much of a predation risk for Nephila spiders in nature. Singapore has large, dominant tree-dwelling ants- including the formidable Oecophylla weaver ants- that likely are more dangerous to spiders and would have presented a more realistic test of the spider’s chemical defenses.

source: Zhang, Koh, Seah, Lai, Elgar & Li. 2011. A novel property of spider silk: chemical defence against ants. Proc Roy Soc B