Woolf seems to desire to compel us to see ourselves in the moth. Thus, one of the first devices she uses is personification of the moth itself. Personification entails the attribution of human qualities to something that is not human. She says that, despite his hybrid nature and the fact that he is "neither gay like butterflies" nor somber like the moths one sees at night, he "seemed to be content with life." To suggest that the moth feels contentment is to personify it.
Next, she explicitly states that anyone watching the moth flit back and forth across the window would become "conscious of a queer feeling of pity for him" because of the evident "zest" he seems to feel in "enjoying his meagre opportunities." Here, Woolf employs pathos, raising our sympathy and compelling us to feel something for this small creature (to whom we might normally never give a second thought).
Having elicited our sympathy for the moth, she uses a simile, saying that "it seemed as if a fibre, very thin but pure, of the enormous energy of the world had been thrust into his frail and diminutive body." This comparison makes the moth seem somehow exceptionally alive, as if his little life force is especially bright and vital.
Woolf continues to use pathos when she describes "the thought of all that life might have been had he been born in any other shape caused one to view his simple activities with a kind of pity." She compels us to sympathize with this little moth who is so clearly full of life and vigor that he might have accomplished much had he been born as some other creature.
Likewise, as she watches the moth die, realizing it is the "oncoming doom" that affects us all, she cannot help but think of how none of us has "any chance against death." But then, when he succeeds in getting onto his feet one last time, "One's sympathies...were all on the side of life." Pathos seems to me to be, by far, the most frequently used rhetorical device in the essay.