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Photographed yesterday in Zhejiang China. Tiny, around 2mm in length, lingering on duckweed. Ephydridae?

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another shot

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While my initial reaction is Hydrellia albilabris, the head is not quite right - H. albilabris should have a more pronounced lunule above the antennae. I wonder whether these are Lemnaphila, but I'm not familiar with that genus, and it's not recorded from Asia. Wait for Tadek to comment...

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A vert enlightening comment! Thank you! This fly looks indeed similar to and shares very similar habitat with Lemnaphile https://bugguide....44/bgimage. Perhaps a new record from Asia of this genus? :)

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Another shot

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first reaction of Tony is correct. According to the present taxonomy it is a Hydrellia from the albilabris group. The species on photographs is similar to australian species Hydrellia williamsi, having much darker legs. The group is rather associated with Cavatorella, known from Japan, not with Lemnaphila, sharing with the latter only superficial similarities and biology (mining in Lemna).
Tadek