Gasteracantha cancriformis is built unlike most orb-weavers. The female's abdomen is broader than it is long, hardened, and carries six conical spines around its edge, in a colour scheme of white or yellow with black spots and darker tips. The male, at two to three millimetres, is a plain small spider.
The standard explanation is anti-predator. A rigid, awkwardly shaped body is harder for a bird to swallow or a wasp to grasp, and the bold colours may advertise that difficulty. Related arguments cover the tufts of silk the spider adds to the foundation lines of its web, which are thought to make the structure visible to animals large enough to destroy it.
Both accounts are inferences from form. Controlled tests of whether the spines actually deter predators are few, and the shape may also relate to the mechanics of carrying eggs. The spider itself is harmless to people.