Brachonid Wasps are a family of Ichneumon Wasps; they have stockier abdomens and a different pattern of veins on the wings.
Like all Ichneumons, the females are parasitoids and may have long ovipositors to lay eggs in nesting holes or bodies of host species.
A female Brachonid Wasp with a long ovipositor on thistle in NW London.