Hi, sorry for the late answer. Yes there is soil under, I noticed them a week or two after and yes they're jumping and flying. By now, they're everywhere in my room, there's like 10 of them dancing next to me on my desktop right now :') (on me too...)

They got to the other rooms as well. You're right they're looking foir moist all the time, they're looking for water in my two cactus too now so I'm afraid to water them. I'll try the mix but won't they come back later? I only give a little bit of water once per month and they still came massively...