Ant lions don't go anywhere near city's. Because of one reason they are surrounded with thumper's and defences. The main reason is that City 17's citadel provides the energy to the nexus of the city's, even if it's destroyed the statis fields will remain up. Ant lions also tend to stay to sandy areas, where they can breed and do as they like. That's the main reason why Nova Prospect fell, because it was on the edge of the coast line.