Spiny orb-weavers is a common name for Gasteracantha, a genus of spiders. 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.
Taken at Kadavoor, Kerala, India
[Thanks Prem (Kuttan) and Ezhil Ramalingam for the ID. Thanks all for your title suggestions.]