In a large microwave-safe bowl, heat margarine and marshmallows at HIGH for 3 minutes, stirring after 2 minutes. Stir in KELLOGG'S RICE KRISPIES cereal until well coated. Using spatula sprayed with cooking spray or waxed paper, press mixture into a 15 1/2 x 10 1/2 x 1-inch pan coated with cooking spray. Allow mixture to cool slightly. Make caterpillar sections, using small round cookie cutter, cut cereal mixture into circles. Place eight to nine sections next to each other to form caterpillar. Decorate with icing to make antenna, eyes, feet and spots on caterpillars.