Apparently, it was a slow day for this particular fly. Typical of its species, it sits on the end of a cactus needle, facing the plant and somewhere in the vicinity of cochineal. This one stayed on the same needle for at least a half hour, as I saw it a second time on the return trip of my hike. Given the very prickly pear that supported the needle, I couldn't get too close, maybe 18" away. As I tried different angles, the fly rotated on the needle.

Another fly I shot did walk down the needle to the cactus blade. I've seen others walking on the blade, typically going from one clump of needles to another, but haven't seen any enter the cochineal.