St Andrew's Cross Spider

The St Andrew's Cross Spiders build medium-sized orb webs, occupied day and night, on low shrubby vegetation.

The role of the cross-like web decoration, called the stabilimentum, has long been a puzzle. At first thought to strengthen or 'stabilise' the web, more recent ideas associate it with capturing prey or avoiding predators. The ribbon-like silk reflects ultra-violet light strongly. Such light is attractive to flying insects, which use it to locate food sources like flowers and to navigate through openings in the vegetation. If the stabilimentum silk attracts insects it may increase the web's prey catching efficiency. The silk decoration could also make the web and its owner more obvious to day-active predators like birds and wasps. However, the variability of the shape of the cross decoration (a complete cross; a partial cross with from one to three arms; or sometimes absent altogether) could make web recognition confusing for the predator. Another possibility is that the stabilimentum advertises a warning to predators like birds to stay away - after diving through the sticky web, the effort required to clean silk off plumage may deter birds from trying again.