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>the thistle belongs to the genus Cirsium.

Typically, Terellia serratulae is associated with Carduus nutans and allied species, whereas T. longicauda Meigen feeds on Cirsium eriophorum. The female on the picture has ovipositor 2/3 as long as in T. longicauda, and almost as long as in T. serratulae.

There are several serratulae-like populations of uncertain status over Europe and Central Asia, which need revision including a lot of morphometry, electrophoresis, DNA-sequencing, etc. to clarify their status. I am keeping to accumulate these data still, so maybe some day...

But so far - it must be treated as T. sp. cf. serratulae.