Habit of caterpillars, feeding on California Sun-cup (Camissonia californica) in a desert canyonette. [Note: The plant in left foreground is different...it's Chuparosa (Justicia californica).]

I believe this is Riding's Forester (Alypia ridingsi), as it matches fairly well with the caterpillar image here. Also, it's feeding on one of the hostplants mentioned here, where there is also a detailed description of the larvae...which agrees quite well with the specimen(s) in the photo (except for the repeated mention of patches of mauve-brown color at various spots, which I'm thinking may not be especially visible in the photos, or simply variable). I had photographed an adult A. ridingsi two days earlier in the same vicinity.

Of course, it cold be something entirely different --- I don't have much experience with caterpillars. Comments, corrections, or suggestions welcome & encouraged.