After several years of looking for this species I finally had some good views and managed some photos. Previously I have seen no more than a glimpse before they fly away never to be seen again! This one (and another species found later) stayed long enough for some shots. I find these insects quite bizarre looking, and also very beautiful. The family they belong to, Derbidae, is apparently mostly tropical, but luckily for us, there are a few species this far north. These ones feed on woody fungi, according to Steve Marshall.