The spider is the poem's centrepiece, and it changes what the poem is about.
Up to that point the discoveries are still: shells sitting in chalk, moss standing like a forest, a drop of water. With the spider the lens catches something at work — “How the deft spider jets / The woven web-silk / From his spinnerets”. The speaker sees not just a creature but a process, and even names the organ, spinnerets, which is a real anatomical term. He also has to be patient for it: “I lie in wait and watch.”
Then, in the very next stanza, the mood turns: “The tigerish claws he has! / And oh! the silly flies / The stumble into his net— / With all those eyes!” The lens has shown him beauty and craft, and in the same instant a hunter and its prey.
Why that matters: the spider stops the poem from being merely pretty. Close observation, the poem admits, does not only reveal wonders — it reveals what nature actually is: skilful, armed and indifferent. The word tigerish puts a jungle predator inside a creature the size of a fingernail, and the flies' many eyes make their blundering worse, not better. It is the one place where the poem's delight has an edge to it.