These are flies from the family Ulidiidae. It seems that people did not really bother to give all of them common names, but they are sometimes called picture-winged flies.
This particular species seems to like to gather on some bracket fungus. I remember seeing these flies at the same location where the picture was taken decades ago, but I never knew what they were.
After I took these pictures I started looking on the Internet and I have identified them from other pictures — which might not be a good way to identify any insect since there are so many of them.
Myennis octopunctata on diptera.info.