Thick, round, black balloons
A few inches of string or yarn - or a twist tie
4 black pipe cleaners
White out (liquid paper) or white tempera paint
Blow up a round black balloon so that it's only slightly inflated. Tie the balloon shut.
Twist the balloon, making a large part (for the body) and a smaller part (for the head). Secure it with a piece of string or a twist tie. If your balloon isn't stretchy enough to do this, use two balloons - use one for the body and one for the head. Tie the two balloons together with their stems.
Fold each of 4 pipe cleaners in half. Twist the middle of each pipe cleaner and wrap around the twist in the balloon. This makes eight legs.
Fold and shape each leg to look like a jointed spider's leg.
Draw eyes and fangs, using white-out (liquid paper) or white tempera paint.