What does the phrase in bold mean?
At once, the spider’s legs bent in upon its body; it rolled over and began to twitch horribly, rocking from side to side.
I think: The spider's legs bent inward on top of its body not under its body. I found this sentence in Harry Potter and the Goblet of Fire
I want to know what this process looks like. (the way its legs are bent, the shape it gets into. Basically, what a native speaker would understand when he or she reads it.)