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Here is one of the "singles" of the Fungus Gnats, and judging by the markings on the thorax it is the same species as the mating pair.

Looking at the wing, though, this one has a small square closed cell near the leading edge that the other photo (at the top of this page) doesn't show.

My other photos have some of the Fungus Gnats with this cell and some without. What is going on?