Thoreau talks with a recluse who lives in the woods, the Hermit, about going to the pond to fish together. Then, Thoreau explains of his thoughts about animals, who, he says, "are all beasts of burden, in a sense, made to carry some portion of our thoughts." He describes the behavior of the wild mice, the robin, the partridge, a cat he once found by the shore, and the loons on the pond, who make "perhaps the wildest sound that is ever heard here." He admires the animals for living wild and free.
For Thoreau, animals are "beasts of burden" not as they are for the farmer, who makes them labor, but metaphorically, because they help Thoreau to work out his thoughts. Thoreau and the Hermit aspire to be like the animals, living freely in nature, though Thoreau further aspires to combine that freeness with a life of the intellect.
Thoreau notices a war between two races of ants, red on one side and black on the other. The ground by his house is covered with the combatants, and he compares them to ancient warriors for their determination and heroism. He puts a couple of them under a microscope and observes their injuries. Thoreau is excited and harrowed as if he witnesses a human battle.
Not all of what Thoreau admires in nature is peaceful, like the war between the races of ants, but all of it has a special grace and beauty akin to virtues of the greatest men. He is not being sarcastic in comparing the ants to ancient human heroes of legend.