Three critical characters for the ID of R. rex are barely discernible here in this (unfortunately more blurry than I'd like) image:

1) the obliquely truncate hind basitarsus;
2) the finely "nub-like" serrations on the anterior edge of the mid-tibia; and
3) the hairy tip of the conical projection at the base of the foreleg, next to the coxa.

In the image the ripiphorid has her ovipositor inserted in the flower head.