St. Andrew’s Cross Spider from Australia

Subject: Spider
Location: Sydney Australia
January 19, 2016 1:56 am
Is that a Nursery Web spider?
Interesting web addition in the direction of the legs.
Signature: Walter

St. Andrews Cross Spider
St. Andrews Cross Spider

Dear Walter,
This St. Andrew’s Cross Spider,
Argiope keyserlingi, is an Orbweaver, not a Nursery Web Spider.  According to the Australian Museum:  “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.”

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