This huge, imposing (and absolutely harmless to humans) big, black fly is the Mexican cactus fly (or Mexican flower fly), Copestylum mexicanum. It is named because the larvae feed on rotting cacti (and the adults, as you might guess from the above photo, visit flowers). This individual is about 13-15 mm long, a very big boi by my standards.
Bugguide’s guide to “identification” for this species says only “Alula entirely covered with microtrichae,” which is so helpful: Googling “Alula” got me a page for a home security system. “Alula fly” got me a resort in Saudi Arabia. “Alula fly microtrichae” got me a page on bugguide where someone was trying to determine if a fly’s alula had microtrichae or not. “Alula fly anatomy” reveals that the alula is a wing bone in a bird. At last, “alula insect anatomy” gets me a NIH article: “The alula is a hinged flap found at the base of the wings of most brachyceran Diptera. The alula accounts for up to 10 per cent of the total wing area in hoverflies (Syrphidae), and its hinged arrangement allows the wings to be swept back over the thorax and abdomen at rest.”
The next sentence starts “The alula is actuated via the third axillary sclerite….” Hoo boy.
Identified via the kind people at iNaturalist.