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27.02.2024 Russia, Leningrad district, Korpikovo
In the well

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2

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Pediciidae genus Pedicia or Dicranota. It all depends on the absence (Pedicia) or presence (Dicranota) of a crown of hooks on the ventral pseudopods. But I guess it is Pedicia as Dicranota is a species living in brooks and rivers

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Atylotus, many thanks!
Apparently, you are right, since there are no hooks on the ventral pseudopods. And since it's so big, I'm guessing it's Pedicia rivosa?

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Yes, this is Pedicia but I can't tell which species. There are numerous species of Pedicia in your area. Savchenko (1986) in Fauna Ukraine 14 gives a key to the larvae only to subgenus but I do believe that several species have not been described in the larval stage. However, big larvae up to about 45mm most likely belong to P.rivosa, at least P.(Pedicia) sp. Smaller ones, of less than 25mm are something else (subgenus Crunobia and Amalopsis). Also, according to Savchenko, the larvae of the latter two subgenera are brownish and Pedicia rivosa white, except for the first couple of segments dorsally.. So, P.rivosa is highly likely in your specimen, but I see some more brownish patches.
Savchenko gives 1 species of Pedicia (Pedicia), 6 species of P.(Crunobia) and 1 of P. ( Amalopsis). Larvae of Diptera in general are always longer than the adult, about 2 times longer. So if your larvae is 4cm, the adult is about 2cm