St Andrew’s cross spider’s cunning ploy to lure prey

Louise King's Long Listen Festival at Eudlo Hall, kicks off September 29.

Nature Watch with Don McGlusky

I came across this specimen while tidying up some scruffy Bromeliads. You don’t often see one as big as this one. The St Andrew’s cross spider is an orb-weaver spider found across much of eastern Australia. It belongs to the genus Argiope, whose members are famous for their often brightly colored abdomens and the distinctive zig-zags they weave into their webs. 

The zig-zags in the web of a St Andrew’s cross spider form a large X shape, similar to the heraldic symbol known as a saltire. It’s also known as Saint Andrew’s cross, since Andrew the Apostle is traditionally said to have been crucified on a diagonal cross in the shape of the letter X. The cross of a St Andrew’s cross spider is woven with a bluish-white silk that strongly reflects ultraviolet light. 

Many flying insects are attracted to UV light, so the cross might lure unwitting prey into the spider’s clutches. Another theory suggests the cross helps protect the spider from predators. When a St. Andrew’s cross spider sits in the middle of the X, aligning her outstretched legs with the arms of the cross, it can make her appear much larger, possibly intimidating potential predators.

The big, colorful spiders are females, while males are many times smaller and less vivid. Its venom is not highly toxic to humans and, like most spiders, it is generally not aggressive with people.