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Very similar to one that John Smit suggested might be Acrosticta apicalis, on this thread:

http://www.dipter...d_id=44452

This one about five metres from the same spot and broadly similar (but without the appearance of a particularly elongate abdomen)... so possibly the same.

Location: Chongwe District, Lusaka, Zambia.
Habitat: Sandy, disturbed ground. Quite a cluster on what were probably guinea-fowl droppings.
Date: 04 March 2013.
Size: Small... 6mm or thereabouts.

UPDATE - title updated from 'Ulidiidae - Euxesta/Acrosticta?', as family corrected by Nosferatumyia