If you are walking along the forest edge, you may encounter a spider’s web with a prominent pure white “X” in the centre. Take a closer look. The entire web may not be very visible, but the central cross is. The cross-shaped structure is made up of zigzag bands of shiny pure white silk. Resting on this cross is the spider, head pointing downwards, legs paired and outstretched to align with the cross (above). Thus the common name, St. Andrew’s Cross (Argiope versicolor), as the “X” resembles the cross of St Andrew. Sometimes the cross is incomplete, with a ‘hollow’ centre where the spider rests (below).
This prominent white cross of zigzag silk is called the stabilimentum. It is only present in the web of the mature female. The web of the male is decorated with a lace-like centre LINK.
And it has always been believed that its function was to strengthen the web or even to conceal the spider. But this theory has now been discredited. It is also believed that the white cross helps attract prey to the web as it reflects ultra violet light, and insects are attractive to this light.
It is now believed that the main function of the stabilimentum is actually to make the web more visible to large animals. Birds have been known to fly through spiders’ webs without stabilimentum but not those with. Such warning is beneficial to the spider as it would reduce the need to repair the web. But then, highlighting the spider would attract those birds that eat spiders, like spider hunters. A more detailed discussion on the functions of the stabilimentum can found HERE.
St Andrew’s Cross spiders are harmless. They feed on insects and in turn are predated by some species of birds.
This discussion arises from Lee Chiu San‘s post of a nesting pair of Olive-Backed Sunbirds’ (Nectarina jugularis) nest building efforts that was stopped by the arrival of a St. Andrew’s Cross spider building its web around the nest- see HERE.
Note: Top image by Johnny Wee, bottom image by Am.
I think the theory about the stabilimentum being “a multi-functional structure that has proven useful for all of the reasons described above” (from the 2nd last link in the article) makes the most sense. However, that doesn’t explain why some spiders do 2 diagonal lines and not the complete “X”. Or maybe they somehow feel that 2 lines are sufficient in their particular context? If so, then the spiders are much more intelligent than we think!