Horned spider Gasteracantha arcuata.

Spiders of the Gasteracantha genus build vertical orb-webs with an open hub which are easily recognised. They have a hard, flat body armed with three pairs of spines on the edge of the abdomen. The spinnerets are elevated on a large projection and surrounded by a sclerotised ring-like structure. Classification: Family Araneidae, Orb-Web Spiders.
Female: 10 mm.
They are also commonly called Spiny-backed orb-weavers, due to the prominent spines on their abdomen. These spiders can reach sizes of up to 30mm in diameter (measured from spike to spike). Although their shell is shaped like a crab shell with spikes, it is not to be confused with a crab spider.