A female Torymid wasp, probably Torymus sp. (Hymenoptera: Torymidae), checking out a smooth spangle gall on the underside of an oak leaf.
Smooth spangle galls are produced by the larvae of the Cynipid wasp Neuroterus albipes. Some Torymid wasps parasitize N. albipes by using their long ovipositors to drill into the gall and lay their eggs next to the larva in its centre. When the Torymid larva hatches it then feeds on the N. albipes larva.
On this occasion the Torymid wasp stayed on the gall only for a few seconds and so presumably did not lay an egg. Maybe it was able to detect that the gall had already been parasitized?
For more photos of these green Torymid wasps see here.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-10-25.