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I saw this smalle yellow fly first time today - in numbers. First one in bramble bush, then 10 on a wooden pole in the Amsterdamse Bos (nutrient rich, on clay, quite wet), then 1 in my garden! Is is small, 2 - 3 mm. It hoovers slowly and if so you see a lemon pinhead moving.

What can it be? And what is this strange structure on the tip of the abdomen? I've seen it in at least two specimens, so it is unlikely to be teratogenic.