Galls of the Silk Button gall wasp (Neuroterus numismalis) on the underside of an oak leaf. A single wasp will emerge from each gall. Neuroterus numismalis has two generations per year, the first sexual and the second agamic. These 'silk buttons' are caused by the agamic generation whilst the sexual generation gives rise to a different form of gall known as 'blister galls'. Peak District National Park, Derbyshire, UK. August.