How does Leiningen know the ants are approaching without seeing them?

Leiningen learns that the ants are approaching his plantation because he is told by a Brazilian official that the ants will probably reach his plantation in approximately two days.
When Leiningen hears the words of the official, he tells the man, "even a herd of saurians couldn't drive me from this plantation of mine." And, despite the official's explanation that the ants are "elemental" and an "act of God," Leiningen refuses to leave his plantation. For the past three years, he has been a planter and dealt with flood, plague, and other "acts of God." Furthermore, he prides himself on how he has triumphed over every difficulty.
Even when he sees the legions of ants that march toward his land, eating everything in their paths, Leiningen refuses to leave because he believes the human mind can surpass the limitations of insects. Indeed, he believes he can win the war against them.

The human brain needs only to become fully aware of its powers to conquer even the elements.

In addition, Leiningen has always known how to "grapple with life," and he is determined to make the most of the fight against the ants.
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