Sepsidae are a family of flies, commonly called the Black Scavenger Flies or Ensign Flies.
There are approximately 250 species worldwide.
They are usually found around dung or decaying plant and animal material.
Many species resemble ants having a "waist" and glossy black body.
Many Sepsidae have a curious wing-waving habit made more apparent by dark patches at the wing end.

Zanderij Crailo, Hilversum, The Netherlands, 22 June 2013