Common name for this fly's family: aphid flies. Family members also prey on mealy bugs and, in the case of this particular Leucopsis sp., scale insects. The fly's larvae are the predators and, besides a satisfying meal of cochineal insects, they benefit from incorporating the prey's carminic acid into their own defensive systems. The fly ejects the acid along with rectal fluid, a substance which ants just hate. (Any ant will tell you that!)