Root borer on Lactuca sp. Unlike the root miners on the same individual plant, who fed just under the root epidermis and left the cores of the roots intact, this fly larva had completely tunneled out the interior of its root, leaving just an empty husk.
Unfortunately I don't have a clear image of the whole larva because I mortally injured it in the process of finding it. The anterior end of the larva was undamaged, however, so I was able to get some pictures of that part of it (see current image).

Host is a rosette of a hairy-leaved Lactuca I haven't identified to species yet.