I found a dimpled spider, fat and white
On a white heal-all, holding up a moth
Like a white piece of rigid satin cloth--
Assorted characters of death and blight
Mixed ready to begin the morning right
Like the ingredients of a witches' broth--
It’s no coincidence that all three of these organisms (the spider, the flower, and now the moth) are  described as white . Frost may be telling us that the cruel predator (spider), the pitiful prey (moth), and the beautiful healer (flower) are all connected in a web of life and may have more in common than they seem. 
 Here, Frost is also using juxtaposition by describing satin cloth–something smooth and silky–as “rigid”.
Design