Central to this poem is the way that the spider, the flower and the moth are symbols of death, destruction and decay. Notice how Frost describes them as "Assorted characters of death and blight" and ingredients of a "witch's broth." However, as the second stanza establishes, the real theme of the poem is to what extent any power or divinity "designed" such an event. Frost seems to be grappling with the question of how we balance such symbols of death with a loving and ultimately caring deity. Linked with this are questions of God's role in nature: how involved is God in such minor events or is He only involved in larger, more important things: the big "design." Frost, by asking a series of questions in the second half of the poem, profoundly unsettles us with these issues, and refuses to conclude anything, not giving us the satisfation of having an answer. We are left mulling over the same questions that Frost asks, pondering how death and life sit side by side and to what extent are things "designed" by a deity.
Thus the symbolism of the flower, spider and moth forces us to confront the central issues of the poem. By connecting them to death and decay we are challenged to think about God's invovlement (if at all) in such events.