There are many species in the Gasteracantha family, of which 4 occur in Madagascar:
G. versicolor, G. sanguinolenta, G. thorelli and G. rufithorax.
I do not known which species this is.

Top & Left: Mananara Lodge area, Anjozorobe, Madagascar, 17 January 2014
Middle: Andasibe-Mantadia National Park, Madagascar, 11 January 2014