This beautiful creature is a large robber fly, one of the Hanging Thieves, likely Diogmites basalis. I took a picture of this presumably ovipositing female who (out of the camera’s purview) was digging her abdomen into a sandy or gravelly part of my backyard. She is sporting quite the handsome mystax (i.e. the white mustache) that serves as protection from counterattacking prey, such as stinging Hymenoptera. A print of this photograph is featured in my Bugs in the Library exhibition at the North Adams Public Library.
[Please note: Second photo features a different individual.]