Something has been puzzling me about the gall flies (Diptera: Tephritidae) I have been seeing this summer. Last year almost all of the Tephritidae I saw belonged to the species Terellia tussilaginis, with just one each from Tephritis matricariae and Chaetostomella cylindrica. This year, until recently, I had seen lots of C. cylindrica, Xyphosia miliaria, Tephritis bardanae, a few Terellia ruficauda, and one each of Urophora cardui, U. jaceana, U.stylata, U.quadrifasciata, and Anomoia purmunda, but no Terellia tussilaginis at all. Or so I thought.
Then, in late July I took the following photo of what, at the time, I assumed to be C. cylindrica on burdock but which, on getting home, I realized were actually my long-lost T. tussilaginis:
This caused me to look back though my photos from earlier in the summer to see if there were any other T. tussilaginis hiding amongst them. And sure enough, I found several going right back into early July, but the surprise for me was that the first photos from July 7th show T. tussilaginis not on burdock but on thistle (Cirsium sp):
I suppose this individual could have temporarily strayed from burdock plants which I remember were only a few metres away. Indeed, when disturbed it flew off and landed on a nearby hogweed plant and I have some photos of it there, and I don't think there is any question that that was just a chance association.
Anyhow the fact that I first saw them on thistles probably explains why I failed to recognise them.
Photos taken in Whiteknights Park, Reading, UK, on 2010-07-24 (first) and 2010-07-07 (second and third).