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Tenthredo mesomelas
I saw this fly or hornet in the forest next to my house (second picture shows the whole thing). Black on the topside of the body and bright green on the underside. Hornet-like face with black compound eyes, green mandibles and short but thick antennae. About 1,5cm in length.
I think this is Tenthredo mesomelas. A common sawfly across Europe. Very similar to the previous suggestion (Rhogogaster viridi) but this species has black markings on its flanks where R. viridi doesn't. :)
Nice creature - I'm not sure about the suggested ID, and didn't find an English name.