I don't need a precise ID, but does anyone know what group these flies belong to? They were enjoying these flowers at the UC Berkeley botanical gardens (San Francisco Bay area). I don't know what the plant is, but it was in the South American section. (I tried to be clever and took a photo of the plant tag but it was obviously referring to something else--this is a large woody shrub and not a Hippeastrum sp.) They are almost 3/4 of an inch long and do not stay put if you stick a cell phone up close. You can't sneak up on them at all.

I took many photos before my family made me move along and none are very good, but I'd appreciate any guesses anyway. Thanks!