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According to Magill’s Survey of American Literature, Revised Edition, at enotes.com, some of the literary devices Emily Dickinson employs in her poem "I heard a Fly buzz —when I died" are extremely sophisticated.
The first device used is called synesthesia. This device involves the use of one sense to describe another. For example, the breaths of the "watchers" are gathered, waiting for the last breath of the woman who lies dying:
And breaths were gathering sure
For that last onset...
The other is called paronomasia which is another term for wordplay, a complicated way to used words in both obvious and subtle ways which may only be noticed by the most discerning reader. For example, the room where the woman is dying is very still, but the atmosphere is charged like the lull between the movements of a storm front:
The room is still, but this stillness resembles the interval between the heavings of a storm.
There is the discussion of the dying person's will, and the will in which she has bequeathed her belongings. The wordplay here is found with the word "will" with its different meanings.
In general, the poem is made up of four stanzas, which act like paragraphs in a poem. It is in the stanza that we find the rhyme scheme of the poem: the end rhyme—or the rhyming of the words at the end of lines—follows the pattern: abcb. This means that the words at the end of the first and third lines do NOT rhyme, but the second and fourth lines DO rhyme at the end. For example, in the first stanza, "form" and "storm" rhyme. In the second stanza, "sure" and "power" rhyme (though "power" is closer to a "near-rhyme," also known as "slant rhyme"). The third stanza shows the second and fourth lines rhyming with "I" and "fly" and so forth.
The rhythmic structure in the stanzas, for the most part, seems to show three stressed syllables in the second and fourth lines (called "trimeter"), and four stressed syllables in the first and third lines (called "tetrameter"). The meter represents a certain number of paired syllables for each line, also called "feet." The stress usually falls on the second syllable. For example, in the following line, see where the stress lies—this stress creates the rocking motion of the poem, especially when read out loud:
I heard a fly buzz when I died
The stress lies on "heard," "fly," "when," and "died." When scanning the line, it would look like this: ^ / ^ / ^ / ^ / (where the caret "^" shows an unstressed syllable, and the slash "/" shows a stressed syllable).
Onomatopoeia is used with the world "buzz" and "breaths." And imagery, another poetic device, is vividly rendered in the line "There interposed a fly, / With blue, uncertain, stumbling buzz..."