Symbolism, Imagery, Allegory
We know that Ron is afraid of spiders – we find out when Harry and Ron head out to the Forbidden Forest in Harry Potter and the Chamber of Secrets and meet a giant spider (hey, anyone would be afraid of that). When Professor Moody brings out his jar of three spiders to demonstrate the Unforgivable Curses, Ron is frightened even before Professor Moody demonstrates the curses. So, in this scene, the spiders are a symbol of fear; they underline how terrifying these curses are by their very nature.