There isn't much left of the wasp abdomen. They must be quite vulnerable when they are ovipositing.
After taking the photo, I barely touched the wasp carcass and it fell to the ground but the ovipositor is still secure.
On this same tree, I did see an Opiliones clutching a Megarhyssa macrurus but after returning with a ladder for a photo, I caused too much commotion and the Opiliones dropped the wasp.

Ah, but I had another chance


Another day I found the last abdominal segment and ovipositor of a female M. macrurus but no evidence of the predator.

the story
Tree of the Year, 2016: a Pileated Woodpecker tree