Why do ants crawl in one straight line?

Ants are very organised. The nest will send out scouts to look for food and each ant leaves a scent trail. Once a scout has found a food source she will head back to the nest leaving a stronger trail. Other ants then follow that trail and, if it's crooked, may take shortcuts which, of course, also leave a trail. In time the path becomes well defined and all the ants will follow that path. I don't know exactly what ants communicate to each other when they meet but it's probably a reinforcing 'Hi, sister! Yes, there's food that way' or something like that. They certainly touch each other with their antennae in a greeting sort of way.