Like many other insects, fire ants use pheromones--odors-- for all kinds of communications.
Fire ants use pheromones:
When there are 200,000 ants in a colony, they don’t recognize each other visually. They recognize pheromones that belong to their colony. This how fire ants identify intruders. If an ant has the wrong smell, the intruder is attacked and killed. That way the colony is kept safe. Each colony has its own odor and each fire ant has absorbed that odor into her exoskeleton.
Once a fire ant dies and begins to decompose, worker ants recognize the ant as being dead because of the change in odors. They carry the dead ant to the graveyard. When scientists put the dead odor on live ants, nest mates tried to carry the live ant to the graveyard. Bet that’s a surprise!