My guess is chemical signal. This is based on my knowledge of there tracking system. When a forager finds food, it leaves a chemical trail on its way back to the nest. Future ants follow that trail to the food source. If they find food, they also leave a trail on there way back, otherwise they don't and the old trail fades away.

Also, given there small size, I can't imagine how they could have an effective central command structure, especially when there are simple protocols that allow each ant to act autonomously while still being highly effective for the hive.