Spider webs are great at catching flying insects, but they’re an inviting target for walking ones. The spider sits pretty in the middle of its home, surrounded by the pre-packaged morsels of the insects it has caught. It’s an all-you-can-eat buffet, and ants should easily be able to raid it. Ants are excellent predators, they hunt in large numbers, and they can negotiate their way along the non-stick parts of the web. And yet, there are very few reports of ants successfully pillaging spider webs. Why?
Shichang Zhang has found one possible answer: some spiders lace their silk with an ant-repelling chemical. Their sticky webs, which so effectively trap some insects, can also deter others.
The golden orb-web spider (Nephila antipodiana) is a giant that can reach several inches across. Its web can stretch up to a metre across, and the spider eats and re-spins it every few days. The spider shares its forest with the small pharaoh ant, which is easily small enough to walk along the fine silk threads of the web. But first, they have to get past the spider’s chemical defences.
Zhang found that the spider coats its silk with a chemical called 2-pyrrolidinone. He took some of these threads and strung them across a box, turning them into bridges that ants would have to cross to reach some bait. If Zhang washed the 2-pyrrolidinone away, the ants would happily traverse the silk. If he left the chemical on, they rapidly retreated at the slightest touch.
The same chemical drove off three different species of ants, including the widespread pharaohs. It appears to be a natural part of the ants’ repertoire: some species use it as part of their alarm pheromones, while others have it in their poison glands. It’s probably used by other spiders too. 2-pyrrolidinone is made from another chemical called GABA, which is found on the webs of at least four other spiders. Perhaps these compounds are part of an ancient defence system that orb-weaving spiders use to secure their homes from intruders.
Only adult spiders and large juveniles use 2-pyrrolidinone. Smaller youngsters don’t bother; their threads are too thin to support the weight of an ant, so they have no need for protection. This suggests that the chemical isn’t a natural part of the silk-making process; instead, spiders actively choose to deploy it based on the risk of an ant incursion. This makes sense: it takes energy to make defensive chemicals, and they’re best saved for times when they’re needed. If this is true, then the chemical should also be found on the webs of other large spiders with thick silk strands – something that Zhang can check in future studies.
Reference: Zhang, Koh, Seah, Lai, Elgar & Li. 2011. A novel property of spider silk: chemical defence against ants. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2011.2193
Photo: bySpiderman (Frank)