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it is definitely a pupal case of a Syrphid, but I can't tell the genus without having a closer look at the tail or ventral crotches. A part of the tail seems to be broken off. Sorry, but I'm not an expert on Syrphid pupae, so it won't help in sending me a detailed photograph. But if you're able see an extra row of small teeth just in front of the last pair of ventral prolegs, then it must be an Eristalinus. Also if you're able to get hold on Tsalolikhin (ed.) 1999. Key to freshwater invertebrates of Russia and adjacent lands then, you'll find many drawings of Syrphid pupae on page 878-932. If not, you can borrow mine (Amsterdam).