This is the larva of the variable checkerspot butterfly (Euphydryas chalcedona). There were several on Road F, in the grassland near Trail 3 and the blue-oak forest.

My ID was implicitly confirmed by Irene Brown, who writes:
"The E. chalcedona larvae in grassland near where Trail 3 meets Fireroad F were most likely feeding on low growing annuals in Scrophulariaceae or Platangcea families. I've found E. c, larvae feeding on low growing Scrophs down the hill south of where the Oxygen cylinders were kept and even farther East near the other branch of the creek that eventually joins the branch that runs parallel to trail littlle b in the canyon and on into Searsville Lake.

"I think there is some Mimulus guttatus in a little swale bh..."