Galls of the Silk Button Gall Wasp (Neuroterus numismalis) on the underside of a fallen oak leaf. A single adult wasp will emerge from each gall. Neuroterus numismalis has two generations per year, the first sexual and the second agamic (reproducing asexually). The 'silk buttons' seen here 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. November