I was in the garden this evening, taking down the washing, when this Ichneumon Wasp caught my eye. Needless to say, I flung the washing at the basket and ran indoors for my camera.
Ichneumon wasps are parasitic; laying their eggs on, near or inside the larvae of other insects or spiders. She will detect the larvae by the smell of their droppings and by sensing vibrations in the wood with her antennae. I was incredibly lucky to have caught this female just as she was driving her long ovipositor into an old branch to lay her eggs in the crack.
You can see her walk up and down the wood, carefully laying one egg after another. The long black sheath used to protect the ovipositor when not in use, helps her to balance. When the eggs hatch, they will consume the host. They will overwinter in the wood and pupate in the spring before emerging.
Update (13/7/12) - David Notton at iSpot has given me a possible id for this ichneumon wasp of Ephialtes manifestator. (Thanks David)