Lightly grease an oven tray. Place rice bubbles in a large bowl, set aside. In a large saucepan over low heat melt marshmallows, butter, sugar and cocoa powder. Cook, stirring, for 2 minutes or until melted. Pour melted mixture over the rice bubbles and mix well.
Cut the liquorice into twenty-four 7cm-long strips. Moisten your hands and shape 1 tablespoon of mixture into a ball. Press 4 strips of liquorice across the ball, make sure they evenly overhang.
Make another crackle round and press on top of liquorice. Use icing to glue the red M&M's onto the spider as eyes. Place on prepared tray. Repeat until mixture is finished.
Refrigerate spiders for 1 hour or until set. Serve.