It is clear from the opening paragraphs of this compelling short story that the ants referred to in the title are not normal ants that we would think of in the West. Rather, these are Brazilian giant ants who represent a massive threat to any plantation farmer such as Leningen. Note how the Brazilian District Commissioner responds to Leningen when he says that he will not leave his plantation:
"Leiningen!" he shouted. "You're insane! They're not creatures you can fight--they're an elemental--an 'act of God!' Ten miles long, two miles wide--ants, nothing but ants! And every single one of them a fiend from hell; before you can spit three times they'll eat a full-grown buffalo to the bones. I tell you if you don't clear out at once there'll he nothing left of you but a skeleton picked as clean as your own plantation."
The ants therefore represent not just the complete and total destruction of his plantation, but also the threat of death, as these ants are described as an "elemental" force that is completely unstoppable. Each ant is described as "a fiend from hell," and they are clearly carnivorous to such an extent that they can "eat a full-grown buffalo" and leave nothing but its skeleton behind. The ants therefore represent complete ruin and also the risk of death itself to Leningen as he determines to stay on the plantation and do everything he can to stop their advance. The stage is set for this classic story which places man against elemental nature in all of its force and savagery.