Anne Rosenthal points out that Spiders & Insects are members of the phylum Arthropoda -- they are Arthropods. Their exoskeletons do not grow -- the Arthropod must moult periodically in order to grow. So this is a discarded Spider exoskeleton. Another one is shown below. (All images to this point were taken on the rare Chaparral Mallow plant on Trail 12.)

The phylum of Arthropods is interesting. It contains the insects; arachnids (spiders, scorpions, and more) & Horseshoe Crabs; centipedes & millipedes; the extinct Trilobites; and Crustaceans. 83% of the described animal species are Arthropods.