Oak trees seem to be host to an unusual number of parasites. Just by casual inspection I have been able to identify six different types of insect galls on the oak trees in the grounds of Reading University. The above photo shows one of these, the silk button spangle gall. Like the smooth spangle gall that, I blogged about here, this is found on the underside of leaves and contains one stage in the development of a type of small wasp (in this case the species Neuroterus numismalis).
Photo taken in Reading University grounds, Reading, UK, on 2008-09-02.