Symbolism, Imagery, Allegory
In his good clothes and fine health [Drouet] was an unthinking moth of the lamp. (7.3)
[The men] looked at [the door] as dumb brutes look, as dogs paw and whine and study the knob. They shifted and blinked and muttered, now a curse, now a comment. (47.108)
Moths and dogs? Way to insult your characters, Dreiser.
Some of Sister Carrie's literary critics have viewed animal and insect imagery as evidence of the influence of Charles Darwin and his theories of evolution on Dreiser's writing. In very basic terms, evolution posits that we humans are a lot closer to the animal kingdom and non-human life than we may have previously believed. So Dreiser's animal imagery, in turn, might suggest the idea that people, just like animals, are driven in part by biological instincts or physiological processes rather than "higher" moral sensibilities or rational faculties.
Of course, we see characters in the novel grappling with what looks a lot like a conscience (as in the scene where Hurstwood is deciding whether to take the money), so this isn't to say the novel is simply indicating that humans are completely ruled by their physical makeup. It offers, rather, another factor to consider in understanding the complexity of human actions. The narrator remarks on this kind of middle ground that humans inhabit:
Our civilization is still in a middle stage, scarcely beast, in that it is no longer wholly guided by instinct; scarcely human in that it is not yet wholly guided by reason. (8.1)
Partly because of the way its animal imagery suggests some of the darker sides of humanity, Sister Carrie is sometimes grouped in the genre of naturalism (pro tip: check out the "Genre" section of this learning guide).