This year I have been keeping a watch out for great burdock in the hope of seeing the burdock gall flies that I saw last year. However, there was no sign of it in the places where it grew last summer and I was beginning to give up hope. Then, a few days ago I noticed a burdock plant on the dam at the lower end of the middle lake. It was infested with black aphids so I got down and had a closer look and then I noticed that there were also little yellow flies on it. On getting my photos back home I found that these flies were probably Tephritis bardanae (Diptera: Tephritidae) which are similar but not identical to the Terellia tussilaginis that I saw last year (see here and here).
I have noticed before that Tephritidae sometimes flap one wing at a time, as if they are signalling by semaphore:
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-06-19.