One of several small blackflies which I came across on the underside of leaves of burdock (Arctium sp): a female Pachygaster atra (Diptera: Stratiomyidae). A female because the eyes are well separated on top of the head:
At first glance they look like beetles that have failed to properly pack away their wings. This species is characterised by the black basal half of each wing and the femurs being mainly black. Note also the thick ridges behind the eyes.
Photos taken in the field to the west of Chazey Wood, near Caversham, UK, on 2012-07-08.