Sialis lutaria is a species within the family Sialidae (Alderflies).
Adults can reach 2 cm in length and can mostly be encountered from May through June on vegetation near slow-flowing streams or close to ponds.
The other two in Holland occuring alderflies (Sialis fuliginosa en Sialis nigripes) are very rare and only live close to clean, fast-flowing streams.

1st & 2nd photo: Nieuwe Keverdijkse Polder, Weesp, The Netherlands, 2 May 2015
3rd & 4th photo: Landgoed Oostbroek, De Bilt, The Netherlands, 11 April 2019
5th photo: Oostelijke Binnenpolder, Tienhoven, The Netherlands, 12 April 2020