Fear
Fear is a central theme in the text. The speaker's characterization of the snake, and the snake's capacity for evasion, reveals overt feelings of fright. This is apparent in the speaker's early shock at the snake's deceptive initial appearance and escape, and his later descriptions of his shortness of breath and chill. The framing of this experience gives it a clear connection with a natural human fear of the mysteries of the animal world.
Nature
The poem is largely concerned with descriptions of nature. The early scene-setting, as well as the detailed (and double-sided) images of the snake, go far in building a full portrait of an environment. It also carefully puts forward the idea that certain parts of nature are essentially unknowable. As the speaker discovers when the snake quickly darts away from him, sight and significance aren't always reliable in this other world. Some creatures evade both capture and understanding. Through the lens of the snake's unique elusiveness, Dickinson is able to reveal that it is part of the side of nature which cannot be properly tamed or explained, only appreciated from a distance.
Awe
In keeping with the fear and nature threads, a substantial portion of the poem deals with the feeling of awe. Dickinson carefully captures the speaker's mixture of unease around the snake and his appreciation for its strange, otherworldly qualities. As it eludes the grasp of the speaker's hand, he feels impressed and surprised at its ability to vanish. With spiritual overtones in his admiration, he perceives the snake as something beyond his full understanding. The snake escapes the speaker both literally and metaphorically.
