Saint Andrews Cross Spider Mindoro Oriental Philippines Oct 2010
Saint Andrew's Cross Spider
St Andrew's Cross Spiders are named for their bright web decorations - zig-zag ribbons of bluish-white silk that form a full or partial cross through the centre of the orb web. Females have a silvery carapace and a silver, yellow, red and black banded upper abdomen with two longitudinal yellow stripes below. The spider sits with the legs in pairs. The brown and cream coloured males are smaller than females (body lengths 3 - 4 mm and 10 - 16 mm respectively).
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 1 to 3 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.
The cream-coloured young spiders make a circular stabilimentum (like a white silk doily) that disguises them well and may also act as a sunshade. As the spider grows the "doily" is gradually transformed into a "cross".
When threatened, the spider responds, either by dropping from the web or shaking it so vigorously that both spider and stabilimentum become a blur, confusing its attacker. These measures don't always succeed, as indicated by empty, damaged webs and the presence of these spiders as food in the mud cells of wasps.
Mating occurs in summer-autumn and can be hazardous for the small males. One or more males sit in the upper parts of the web - some may be missing legs, survivors of encounters with unreceptive females. The male constructs a mating thread within the web, onto which it attracts a receptive female by vibrating the thread. The female suspends its pear-shaped egg sac in a network of threads, often among leaves where the sac's greenish silk disguises it. Despite this, the egg sacs are often the target of parasitic wasps and flies.
Human Interaction / Threats
No threats identified.
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