This fly hung around the pond. It is a male marsh fly from the genus Tetanocera . The fly larvae are parasitoids on snails and slugs .
References1 Nederlands Soortenregister
2 Barker, Gary & Knutson, Lloyd & Vala, Jean-Claude & Coupland, James & Barnes, Jeffrey. (2004). Overview of the biology of marsh flies (Diptera: Sciomyzidae), with special reference to predators and parasitoids of terrestrial gastropods. 10.1079/9780851993195.0159.