Spiny Orbweaver

Surreal, Kate.

bayucca, I've just received a copy of "Spiders of the Savanna Biome" published by the South African Agricultural Research Council (2013), and this has a photo which matches this one nicely. It is identified only as Isoxya sp., so it might be a currently undescribed species.

Yes, and it could be that some of ones ID as the Isoxya tabulata could also be another one. All the Isoxya tabulata do not look the same.

There are apparently 6 species of Isoxya in southern Africa, and I've only ruled out two of them, so it could be any of the other four.
http://www.biodiversityexplorer.org/arac...
I do have photos of what looks like a much better match to Isoxya tabulata, which I will upload.

... but not quite right ...
Yes, that's best to describe our dilemma.

Thanks, bayucca. I found the same page as your first link - this looks like a good match, but no definitive ID there, unfortunately. From looking at the links, it might well be Isoxya sp. rather than a Gasteracantha, but not quite matching any that I've found. Isoxya tabulata looks close, but not quite right.

I agree with Gastercanthinae, but I am sceptic about being a Gasteracantha. But I am also not really happy with my other candidate. Box Spider, Isoxya sp. and could be Isoxya tabulata Biscuit Kite Spider, but not a typical one. The grooves are too "deep" and strong in yours compared to Isoxya tabulata. But the regularity of the spikes and the box-like or sharp rectangular shape makes it quite unique compared to the other Gasteracantha sp., from which there should be only 4 species in RSA and none matches only nearly. There might be other, maybe rare and not common Gasteracanthinae in RSA. And I am only sceptic, I would not completely exclude a Gasteracantha sp.
Same one like yours, but with my "sceptic" ID. Note also the bicolored legs in yours and Isoxya sp.
http://www.inaturalist.org/observations/...
https://www.projectnoah.org/spottings/34...
http://www.sonyuserforum.de/forum/showth...
http://www.djaniwildlife.co.za/wildways-...
http://saphotographs.blogspot.ch/2013/02...
http://www.ispot.org.za/node/198028
http://www.ispot.org.za/node/184428
http://www.ispot.org.za/node/144528
Here I am more happy with the grooves...
http://www.thefeaturedcreature.com/2012/...
http://4.bp.blogspot.com/-tiBvwGI4ju4/UG...

you're welcome kate ;) i really hope you can get a more accurate Id to this beauty :-) good luck!

Thanks! I've sent the photos to a spider expert in South Africa, so should be able to get a more precise identification at some point.

there are tons of subspecies you'll have to decide which one ;) or get bayucca to help you, or another spider expert