When millipedes are or feel threatened, they will curl up into a spiral and stay still, as the Rusty Millipedes or Rust-coloured Millipedes, seen here.

When doing so, they actually lay on their side, with the head in the centre of the coil, though few animals will eat them due to their hard exoskeleton.

However, though curling up may offer some protection against certain threats, it still leaves them rather exposed to some others, such as being stepped upon by humans.