Scientists returned to the Amazon rainforest in December to collect data on one of their biggest finds of 2012: a spider that uses insect corpses and jungle trash to build big, spider-shaped decoys in its web.
But these Peruvian spiders, presumed to be a new species of Cyclosa, are not the sole sculptors of false arachnids. A second decoy spider lives in the Philippines, on the island of Negros. Finding two spiders that make such similar designs, 11,000 miles apart, has left scientists wondering how the behavior evolved and if the decoys serve as lures for prey or as an anti-predator defense system. The discoveries also suggest there may be even more sculpting arachnids.
You just have to know what to look for.
It is interesting that none of the commentators on the original article mentioned the cause that immediately came to my mind: Because that is the way God made them.
That works a lot better than trying to do some sort of learned behavior/genetic code explanation.