#1
I saw this fly 2 June at Amstelveen, the Netherlands. Several of them sat on a bare tree trunk of a dead willow. This tree was rife with nesting holes of various wasps, the saw dust covering the plants below the tree. One hole is visible on the photograph. The tree was close to a shallow lake. I expect the flies to be nest parasites of the wasps, albeit the fly looks much larger then the holes!. They are medium sized flies, say 8 - 10 mm. Is an ID possible?