Very large wasp. About 1 1/4" long. All black body with orange wings. It landed near my porch then crawled under it, as if after prey.
Fly was on a rock while walking on the red dot trail up Great Blue Hill. I took the suggestion from the website.
Emerging from a Purple-crested Slug (Adoneta spinuloides), whose shell is left behind. The Last Great Act of Defiance of this unfortunate caterpillar appears to have been delivering its last piece of frass.