From Wikipedia:
Sciaridae is a family of flies, commonly known as dark-winged fungus gnats. This is one of the least studied of the large Diptera families, probably due to the small size of these insects and the difficulty in specific identification. There are currently around 1700 described species but there are estimated to be up to 20,000 species awaiting discovery, mainly in the tropics. More than 600 species are known from Europe.
These gnats are small, from one to at the very most seven millimeters long. They have a slender, darkly colored body and dark wings. However, the females of several species are wingless. Their long legs and antennae with 8 to 16 segments are typical of many gnats.

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