This is Delphinia picta, a picture-winged fly. The fly gets its common name from its striking wing pattern; it has two white triangles on the leading edge of each wing and a few decorative white swirls on a shiny brown background. Very attractive!
Picture-winged flies lay their eggs in rotten vegetation. The larvae spend a few weeks feeding there, then pupate for a few weeks, and then emerge as adults. When the weather cools, late season larvae crawl into the ground and become quiescent for the winter. They following spring, they wiggle upward, pupate, become adults, and start the cycle again.
I saw numerous picture-winged flies sunning themselves on benches in Brooklyn Bridge Park this summer (I’m assuming they were sunning, but maybe they were speed-dating). The next generation is surely sleeping under the grass now, not to be seen again until next spring .