I have discovered a very (very!) small ant that was apparently living close (or in?) a nest of Formica fusca. I saw it when I lifted a stone, it was just there in between the other ants. Measured it: 1.5-2 mm (between 1 and 2 mm but closer to 2). The colour is yellow/orange, reminded me of Lasius flavus, just way smaller. It was not a pharaoh ant, it looked just like a Lasius flavus (colour, body, everything, except the size) as far as i could see (though my eyesight is pretty bad). I thought it could have been Diplorhoptrum fugax seeing as it was living practically in the other ants' nest but the body just didn't look like it.
The nest was found under a stone lying on yellow/whitish sand at the edge of the woods.
It could be Solenopsis fugax. You dont have pictures, right ? That would be more helpfully :)
No indeed, I let it go it's way. I cannot find anything of the same species anymore... my guess is they live underground (and never go up). Judging by pictures from google the ant I saw could very well be Solenopsis fugax. Never seen one of those before ;)
I also would go for Solenopsis fugax or Thief ants. Its well known they live with other species in the same nest or they nest with the other species.