Normally, the colour white is associated with life and innocence. In much of Christian iconography, white is the colour of purity, used for christening gowns, altar linens, Easter, etc. Darkness symbolizes evil. In this poem, however, the white images (spider, dead moth) against the dark background are not images of hope or light, but images of cruelty. Both the white spider and the moth are elements of a witch's brew of night creatures, and emblems of "nature red in tooth and claw" in which creatures exist to eat and be eaten.
The poem reframes the tradition questions of theodicy and the creator's design of the world from the more common one of how evil exists or the place in the divine plan of the death of an innocent child, to the question of the theological implications of the death of a moth. The moth's wings conveys images of the claim that God watches even the sparrow fall -- does God watch too over the math which is even smaller than a sparrow but also winged? or does God leave moths to the design of Satan (and the appetite of spiders?)