The common name for this fly's family (Chamaemyiidae) is aphid flies, but family members also prey on mealy bugs and, in the case of the Leucopis sp. shown here, cochineal scale insects. The fly's larvae are the predators and, besides getting a satisfying meal of cochineal insect, they benefit from incorporating the prey's carminic acid into their own defensive systems. Subsequently, the acid is discharged with drops of rectal fluid, and the resulting formula keeps ants at a distance.

Typical of their species, the tiny flies like to sit on the pointy end of a cactus needle, facing the plant, often staying put for a long time. A fly might walk down the needle to the cactus blade, then go for a stroll there. I've seen them proceed from one clump of needles to another, but have yet to see one enter the fluffy white cochineal.

Ron Hemberger