This shiny little marble is an adult female Ornidia obesa (most likely)…a green hover fly, or syrphid fly. (Like horse flies, female green hover flies have widely separated eyes while males have contiguous eyes, giving them a wrap-around sunglasses look.) I find these little things charming, and they’re apparently fearless: this one hung around to get a good look at herself reflected in my camera lens, allowing me to take a couple shots.
These widespread insects are native and common all over tropical areas of the New World.
Both adult fly and maggot are generalist feeders, performing the very useful but not very appealing function of consuming rotting organic matter, sewage, and human and animal feces. I have seen several web pages note that “green hover fly maggots can convert coffee-production waste into a protein-rich material useful in cattle feed” (how did people discover this?).
Green hover flies are mimics of orchid bees, which have stingers while the flies don’t.