I don't think they're symphylans... mostly because they don't appear to have antenna. Yeah, I was wrong about the mosquito dunks. Went down a rabbit hole trying to think of something Belgium might commonly have with Bt in it and lost track of the question. My bad.
Another strangeness here... the photo from OP was uploaded four years ago. Hmmmm.
---------- Post added at 08:08 AM ---------- Previous post was at 08:07 AM ----------
Obviously I was asleep at the wheel.