The Spider is located in the Barrel Room on the 2nd Floor.
What it does
If you don't replace it the door will lock itself and after some seconds a huge spider appears and bites off Viola head, leading to a Game Over. To avoid it you must simply replace the Butterfly with the Butterfly Model as said on the note "Spiders have poor vision" "They cant even tell colours apart."
In Extra mode, when your leaving the Barrel Room with the Butterfly you must run since the spider starts to chase you to the door.
If you Backtrack and try to enter the Barrel Room, the Spider will bite off your head as soon as you enter leading to a Game Over.
When the door locks itself and the spiders comes towards you, run as fast as you can towards it, you can then see in the corner Ellen is sitting on a barrel. That is a memory from when you were a witch killing people you see.