It might not be pretty, but look... It's pretty amazing! 😍💚 I think it was eating the sooty mildew off the leaves.
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A large fly (tachina fera) facing the camera. It has a pale whitish face, big brown eyes, and a yellow/brown stripey body with a black stripe down the middle. It's on plum tree leaves, which on close inspection have some sooty-looking dark patches.