I photographed this insect (~6-7 mm long) in my backyard late yesterday afternoon. Later I posted its picture on BugGuide.net and called it a "red-headed beetle?". Less than 40 minutes later it got identified as Poecilocapsus lineatus, the four-lined plant bug. As its name implied, it was not a beetle (order Coleoptera), but a bug (order Hemiptera) in the family Miridae.
My confusion was due to its unfamiliar wing morphology. I had assumed the yellow and black part was the wing cover (the elytron of a beetle) and the membranous part, which I thought was sticking out from underneath, was the hind wing. It turned out that the entire thing was the wing itself. The figure below explains it.
The wing of a mirid bug. From Gullan & Cranston, The Insects, 3rd ed., 2005.
According to Swan & Papp (The Common Insets of North America, 1972), these bugs feed on the leaves of several plants, including phlox. We do have several phlox plants in the backyard, although when I saw this individual, it wasn't on one of them.