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One of the species of Dasymutilla, but not D. occidentalis (which is an eastern species and doesn't occur in California). There are 3 species with very similar appearances as a part of a larger Müllerian mimicry ring: D. aureola, D. californica, and D. coccineohirta. The head may not be broader than the thorax, so that may be more suggestive of the latter 2.
I'll second that mayflies definitely are not antlions, though that common name gets tossed around quite a bit. In Texas, it's common for crane flies (Tipulomorpha) to be called "mayflies" for some reason. This genus is often noted as picture-winged antlions for their stunning wings, easily the most unique in North America.