You are either thinking of an ichneumon wasp or a robber fly. Both are harmless to humans. Robber-flies are identifiable by the way they hover about when they fly. Ichneumon wasps are identifiable by what looks like an extremely long stinger on their back end, but it's actually a large radar in a way, because it identifies maggots inside of a piece of wood and drills through the wood to get them. Then the maggot is fed to the young.
