Thoreau manages to include several meanings in his brief reference to the Myrmidons, the warlike race who, according to Ovid in the Metamorphoses, were changed from ants to men by Zeus in order to repopulate the island of Aegina. First, there are the generally accepted characteristics of ants, which are portrayed in myth and literature as hardworking, conformist, and acquisitive, toiling together to store up as much wealth as they can. There is also the simple fact that an ant is so small. As Jonathan Swift showed in Gulliver's Travels, the quickest and easiest way to make people and their ambitions seem paltry is, quite literally, to cut them down to size.

The characteristics of the Myrmidons are derived from those of the ants. Although Homer does not mention the legend in which they were created from ants, he portrays them as efficient, conformist warriors, who do as they are told. In "Civil Disobedience," Thoreau remarks that there would be no wars if people would only follow their own consciences and not follow orders mindlessly like ants or Myrmidons. The ants, therefore, are small and petty, greedy for gain, conformist, and mindlessly warlike, all attributes which Thoreau detests. He regards them as the worst possible example for free people, who are capable of living thoughtful, peaceful, nonmaterialistic, self-directed lives.

In Walden, Henry David Thoreau set out to live a minimalist lifestyle in nature in order to pursue happiness and fulfillment outside of the business of modern society. This passage about men not having to be ants is an allusion to Greek mythology, specifically a story where Zeus turned a colony of ants into a race of human beings known as the Myrmidons. Ants tend to be associated with hard work and unquestioned conformity: they are always building nests, gathering food, and defending their home. One ant also heavily resembles another—there is no individuality among a colony of worker ants.

Thoreau feels that modern society urges people to be like ants and "live meanly." Everyone must act like everyone else whether it makes sense to them or not, and everyone is encouraged to work hard to gather as much material wealth as possible, even though material goods do not give life meaning. Such an attitude is not conducive to happiness or mental wellbeing, which is why Thoreau is urging his readership not to conform mindlessly to this lifestyle. To be a human being is to be an individual, according to Thoreau. It also means to dedicate one's life to more than just work or material gain.

One of Thoreau’s main points in Walden is that the lives people usually live are filled with hard work but little joy or meaning. We often pursue the path we do because it is conventional, not because it fulfills us. For this reason, Thoreau compares humans to ants, alluding to a myth from Greece in which Zeus created a group of men called the Myrmidons out of ants (Thoreau establishes his literary credentials, as people did at the time, through many references to Classical literature).

Thoreau has looked around at the life of his neighbors. Many farm, many work in professions they don’t enjoy, and many go into debt to buy more material goods simply because it is what everybody else does. In this sense, they are like ants: they have to toil endlessly, and they are no different from anyone else. By living meanly, Thoreau doesn’t mean they are cruel to others, but that they live threadbare, constricted lives, even if they have piles of stuff stored away, because they are not being true to themselves. If they were, they would move from being like ants to behaving like men: thinking for themselves and becoming fully human.

Thoreau went to Walden Pond as an experiment. He wanted to see if he would be happier living as simply and frugally as possible without having to work more than six weeks a year. He discovered joy and contentment through simplicity, so he wrote his book to persuade other people of the value of his path and to show, with financial details, how people could pursue the simple life. We don’t have to live toiling and faceless like ants, he asserts, but can be who we want to be.

Thoreau is referring to the fact that in modern society man unthinkingly allows himself to be a small part of a large collective, just like a worker in a giant colony of ants. And to a staunch individualist like Thoreau that's a pretty sorry state of affairs. He wants us all to affirm ourselves as unique, distinct individuals with the power to think for ourselves and lead the kind of lives we really want to lead instead of blindly going along with what everyone else is doing.

The fable to which Thoreau refers is from Ancient Greek mythology, when Zeus created a race of people called the Myrmidons out of a colony of ants. Thoreau uses this fable to illustrate his point that modern man is reverting to an almost animal-like existence in willingly subjecting himself to the dictates of the collective.

If you look at the definition for "meanly," you will find that it can mean "inferior" or "of little consequence."  Think about the picture that Thoreau creates with the simile comparing man to ants--ants have no higher rank or consequence over another ant; they each do the same repetitive work over and over, without thinking.  

Thoreau gives the reader this simile to discuss how man treats life.  In the same section he says, "Our life is frittered away by detail."  Man allows all of the "things" to get in the way of living an actual life, which is why Thoreau wanted humanity to simplify instead.  When we rid ourselves of all the unnecessary things in our life, when we think about all we feel we have to do instead of focusing on just what is needed, then we are allowing life to take over instead of living it.