The small reddish sphere attached to the oak leaf is a detachable leaf gall. Galls are vegetative structures produced by some plants in response to some foreign organisms, in this case the larva or juvenile of a small wasp. Basically, the plant creates a specialized structure in which the larva lives and develops but is isolated. A number of different organisms, not always a wasp, cause galls on oaks. If the gall has a small hole in it, chances are that the wasp has matured and left.
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