הזיהויים של haoyuezzz

I saw these flies chasing each other on the trunks and branches of Juglans mandshurica bathed in the sunshine. It seems that they were mating in an unusual way, with the male's front legs touching the female's tail. At first I thought they were robber flies. But after checking the wing vein, I thought they must be something else, most probably something new for me.