I observed an ant trail through a house for some years. I wonder why the ants established that trail.
The ants are small ants of common type for middle europe, found almost everywhere in this region.
The very small trail (about one ant per second) enters the house through gaps near the front door, travel the entire house almost hidden except some short parts where they need to cross nacked floor and exit the house through gaps on the other side. They pass the kitchen, but do not enter it, missing the easy chance of picking food there. Before and after the house, the place is sealed (eg. asphalt or stone plates).
Ants are passing the trail in both directions.
The trail is almost impossible to fight. If cleaned up (using alcohol to remove the traces) it reestablishes in about one day. If holes are blocked, new ones are made (even through silicone or soft concrete) in some days. If hidden gaps are sealed, they tend to use visible trails for some days, but work hard to repoen the hidden gaps. If all ants on the trail are killed or collected over some time, it doesn't seem to have a long term effect at all.
I've never seen any of the ants visibly carrying anything, if the carry something it must be in their stomach.
So why are the ants so eagerly establishing this trail?