Bibio marci is actually not a fly but a mosquito.
They are 8 to 10 mm long.
The family has several species, but the most common in North-West Germany are Bibio marci, Bibio hortulanus and Dilophus febrilis, which can appear in great numbers in the spring.

This mosquito is characterized by a strong hairy glossy shape and short thread-like antennae.
Males have very large, round eyes and completely clear wings.
Females are slightly larger and clearly have a smaller head with rather small, separated eyes and dark colored wings.

Remarkable are the long legs that dangle under the slow-flying mosquitoes.
They are often found in copulation for hours on sunny walls and window glass.

A male. Eifel National Park, Dreiborn Plateau, North-Rhine Westphalia, Germany, 5 May 2022