Secondary screwworm fly (Cochliomyia macellaria), adult, Mount Davidson Park, San Francisco, California - 15 September 2012. I was happy to find this blowfly (Calliphoridae) with a striped thorax on Baccharis (?) - thanks to v belov and Bug Guide for the identification. These flies are "very common and widespread" in California according to Cole (1969). They lay their eggs in carrion or sometimes open wounds and the worm-like maggots emerge and eventually become adult flies like this one. Unlike the "primary" screwworm (Cochliomyia hominivorax), the maggots are not known to invade healthy tissue. The primary screwworm fly was a significant medical and veterinary pest and has been largely eliminated in the western hemisphere due to releases of large numbers of irradiated sterile males.