She is not ovipositing. She is filling her "sand chamber" or "psammophore", a ventral abdominal pocket into which the eggs are laid, and a feature unique to "higher" Bombyliidae. It is said that eggs are coated with sand in the chamber to fascilitate release later. I think that, given the relative size of sand grains and Bombyliid eggs, the reverse is true: the tiny eggs coat sand grains in the chamber, which are heavy and easier to toss with accuracy, kind of like sticking a piece of gum to a baseball. Ovipositing looks like little aerial dipping movements accompanied by flicks of the keester. Eggs are thrown through the air.

Moved from Bee Flies.

moving to genus.