Shichang Zhang and colleagues have found that at least one orb weaver, Nephila antipodiana, deposits a substance on its web threads that deters ants. Ants prey on spiders, but ants have not often been seen preying on orb weaving spiders. Zhang et al. may have discovered why.
Alex Wild, over at Myrmecos, has a good summary of the study as well as an interesting quibble about the range of ants tested.