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I found this large (ca. 18-20 mm) larva vigorously attacking a large earthworm. It curled around the rear end of the worm and punched its skin. It walks quickly and has pseudopods.

I brought both worm and larva home. There, the larva appeared no longer interested in the worm, but had its bellyclearly filled with its victim's blood. The worm is badly injured but still alive.

Which fly could this be? Muscidae. Caliphoridae, Sarcophagidae?

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worm has lesions / constrictions at the rear end;

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blood in the larva's gut

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I know that Pollenia (Calliphoridae) are parasites of earthworms, but I don't know is this one belongs to this genus.

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Thanks, Andrzej;

i know that Pollenia are endoparasites. However that does not necessarily exclude that they might also take a blood meal after their endoparasitic stage - I don't know if these pupate inside their host.

I think Sarcophagidae is the best bet, but who knows?

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Looks like it has a head capsule, so Rhagio is more likely. They are known predators of earthworms.

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Thanks Tony,

I hadn't thought of that option. I'll try if I can make a pciture of the head.