What is the name of the thorn tree that lives in the Savannas?

The most common large thorn tree in southern Africa is the Umbrella thorn tree (Acacia tortilis). An example of the leaves and thorns can be seen in this image, which also shows a pupal case protected by a 'cage' made from these thorns. Image: http://i42.Photobucket.com/albums/e341/Wizzard30/Nature/4505-800.jpg