These spiders get their name for the way they hold their eight legs in pairs to form an X shape. The X is called the St. Andrew's cross because it is believed that the saint was martyred on a cross of this shape rather than the conventional + shape. Besides their standard orb-web, Argiope spiders build additional white opaque zig zag lines on their webs, called stabilimentum. Sometimes the zig-zag lines match their leg positions, which lead some people to suggest that this helps give the appearance of longer legs.
Some Jumping Spiders assume the appearance of an ant by having long and slender legs and what appears to be a three-part (head-thorax-abdomen) body of an insect. To add further to the deception, the fore-legs are often raised in the air like a pair of antennae.
(Dairy Farm Nature Park - 11 Nov 2009)
Some scientists believe that by mimicking ants, the spiders deceive their ant-models and prey either on the ants themselves, or on the homopteran bugs "tended" by the ants. However, it should be noted that the ant-mimicking Jumping Spiders in Singapore have never been observed to have attacked the ants they imitate. A more plausible explanation is that by copying the physical appearance of ants, the ant-mimicking Jumping Spiders are actually buying insurance for self-protection, since spider-hunting wasps, birds and other spider-predators generally avoid ants which secrete the distasteful formic acid when attacked.