When sighting my first Oak Apple Gall, I was bewildered by such a mysterious object. On further handling I found this tan-colored, crisp, papery sphere to be lighter than a ping-pong ball (top photo).
Time spent in a library revealed that the apple gall is caused by several species of gall wasps. The female wasp inserts her eggs into a vegetative bud of an oak tree's branchlet, activating the oak into forming a protective structure, the gall, around the eggs.
Developing galls are spotted green in color (bottom photo). A juicy, white, spongy substance inside the gall surrounds the hard center where the parasite is located. When the parasite leaves, by drilling a hole in the outer wall of the gall, the gall dries and fades in color with the insides becoming a mass of fibers.