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Hello! These Diptera were photographed in small streams in Atlantic Forest, São Paulo State, Brazil. Serra da Cantareira Park - Núcleo Engordador. There were swarms of these insects hovering in the water surface, together with Gerridae and Velidae.
I think they are Empididae, but I didn´t found papers describing this behavior, they are common just above the water, but I never found any mention of a really pleustonic species. It seems that they have hydrophobous fur in the legs.

Can anybody confirm the ID? Any information about this behavior?
Thank you very much!!





A video: https://youtu.be/...