Cut the 1-1/2-inch plastic foam balls in half using a serrated knife. Lay the flat edges of the heads on a cutting surface. Using a crafts knife, cut out a curved section from each head so the head curve smoothly fits the body curve.
Insert a toothpick halfway into each head and centered in the neck curve. Push the rest of the toothpick into a body, aligning the flat sides. Add glue to secure the heads to the bodies; let dry. Paint the spiders with black acrylic paint; let dry.
For eyes, push the ball-head pins into the heads at the front.
Cut the chenille stems in half. Push one end of each leg into the underside edge of each spider body and bend outward. Drop a bit of glue on each leg where it joins the body. After the glue has dried, bend each leg at the knee and foot.
Cut two 9-inch lengths from the black feather boa for each spider. Glue one length in a spiral on the back of each spider and the other length on the tummy of each.