Mystery P-toid

A couple days ago I noticed that one of the beetles I had brought back from the Imperial dunes wasn't looking too healthy. Thinking that it was just reaching the end of its life due to natural causes, I stuck him in a kill jar. When I took him out to pin today, this is what I saw.

Unfortunately the larvae, which I think is a Tachnid, was already dead. This beetle, Eusattus dilitatus, spends almost its entire life underground. In fact, the only specimens I've seen above ground were those that had already perished. They live among the roots of various plants in the Imperial dunes. Check out some cool action shots of this beetle here.
This raises some interesting questions. Do the adult flies just wait for one of the beetles to sporadically surface? Do the adults lay eggs at the base of plants and early instar larvae then somehow search out suitable hosts?
The larvae itself is big, relative to its host size. Imagine living with something that big wriggling around in you!