This afternoon I was busy photographing hoverflies feeding on yarrow flowers (Achillea millefolium) when I noticed this small fly flashing its wings. It is a gall fly (Diptera: Tephritidae), probably species Chaetostomella cylindrica (based on pictures here and here).
Chaetostomella cylindrica is known to parasitize thistles and knapweeds, and I have not been able to find any references to it parasitizing yarrow. However, there were lots of thistles in the area where I took these photos, and it was quite a windy day, so maybe this one had strayed.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-08-16.