On a damp, overcast day in mid-summer I came across this tiny fly doing a frantic random walk over the surface of a leaf. I think it was probably a Platypezid fly (Diptera: Platypezidae). Most Platypezid flies have slightly enlarged tarsi on their hind legs, but my photos are not clear enough to show these, so I have based my tentative identification on the wing venation pattern being similar to that shown here. The larvae of Platypezid flies grow in fungi.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-07-19.