I think it is one of the silly things that Duolingo leaves to be done. A more logical phrase it would be: the ο ιστός της αράχνης. But we don't know the word ο ιστός=the web yet, do we? :)
Notice that I met this phrase in a strengthening exercise.
Maybe "spider" is a person's codename, or something alone those lines!
Our team does, sometimes it's nice to create some unconventional sentences, so that the learners get to learn these new words faster ;)