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Barcelona 30-6-19.Spain.

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Hi, could be Gaurax sp. (Chloropidae) and they are eating - some already being well-fed.

Eric

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To me it looks more like Thaumatomyia. Perhaps they are sucking honeydew. Larvae of this genus are associated with aphids.

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I would say Gaurax, which has been regularly recorded feeding on bird droppings

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Thaumatomyia should have a flat scutellum but the shine on several specemens shows the scutellum is convex. I go with Gaurax fascipes.

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Gaurax Loew, 1863 spec. (Chloropidae). This is a highly interesting photo, as Gaurax species are hitherto not known to aggregate on a feeding source. In this case it may be a bird dropping containing electrolytes which are luring especially tropical Chloropidae (and other flies) in electrolyte-poor habitats and rainforests. Gaurax species are difficult to identify because of their wide colour variability. The male genitalia are specific. Several European species are still undescribed, mostly caught on or reared from dead wood or mushrooms and preferring shady biotopes. If any male could be obtained from the same locality, the writer could identify this species possibly being a first record for Spain.

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Sorry for not answering before. I will try to return to the place and look for these mosquitoes again. I will also look in my file from that day and try to put a more enlarged photo, where you can see more details.
Regards.

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I attach a couple of images. If you want the file in a large size, you only have to ask me and I will gladly send it to you by email.