It is not a catipillar it is a worm that has spikes on its back when you killed it the spikes went back into its skinPLEASE answer this question im getting scared?
Could it be Asian ladybug larvae? They have spikes sticking out of their backs. I don't know about when you kill it the spikes go back in. They just squish when I kill them. Look them up on the internet so you can see a picture.