This line introduces the two sides of this theme. There’s a dying person, and there’s a fly. In a way, this poem is all about the encounter between the two of them. The speaker is dying in the human world, surrounded by other people, but the fly appears from the natural world. It doesn’t know or respect any rules about how people are supposed to die. Just like death, it is natural and unplanned. It’s something that people can’t control. This struggle between natural forces and human beings is a big part of what drives this poem.
Between the Heaves of Storm – (line 4)
Here the natural world is used as a metaphor for something that is happening in the human world. The emotions, the voices, the crying of the people in this room are compared to the heaving of a storm. When the poem opens, everyone has quieted down, and that quiet is compared to the "stillness" that comes during a quiet moment in a storm. Even though there isn’t really a storm going on, this line is still charged with the energy of the natural world. There are powerful forces in this poem, almost a kind of electricity in the air. The way Dickinson chose to express that was by pulling an image from nature and tying it to the human world.
There interposed a Fly – With Blue – uncertain – stumbling Buzz – (lines 12-13)
Can you feel how different this description is from the other images in the poem? Everything else is calm and cool and quiet. The room is full of sadness and a kind of patient waiting. Then nature interrupts in a big way. This little fly buzzes in, totally uninterested in what is going on around him. He doesn’t move calmly and respectfully, but just bumbles around the room, the way any wild creature would, starting and stopping whenever he wants. This doesn’t just interrupt the moment; it changes the whole drift of the poem. We thought we were dealing with a poem about dying, but we were in for a surprise from a fly, and so was our speaker.