As described by Janet Long Nakamarra
The symbol in the center represents the tree of the witchetty grub. People would go out to look for witchetty. They would come to the tree and look for crack marks on the ground. Once they find the marks they then stab the ground with the digging stick to see if the ground is heavy, they then start to dig to the roots. When they find the root is big it tells them the witchetty grub is in the root.
They then break the root with the digging stick and get the witchetty grub out. Some time you have to be careful with the witchetty, if you try to break the root very hard, then you'll squash the witchetty, once its squashed then you just have to eat it.
There are two different kind of witchetty grubs. One that you can get from the root and one that you can get from the branch of a ghost gum. They are two different colours as well. The one you get from the root is white and the one you get form the branch is pink. They both have different names.
Wapilingi - pink witchetty
Ngarlkirdi - white witchetty