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 the Netherlands 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.

1st & 2nd photo: In my yard, Hilversum, The Netherlands, 28 April 2021
3rd photo: Diependal Nature Reserve, Oranje, The Netherlands, 16 April 2020
4th & 5th photo: Oostelijke Binnenpolder, Tienhoven, 12 April 2020