Cut both plastic-foam balls in half using a serrated knife. Lay the flat edges of the heads (cut from the 1-1/2-inch-diameter ball) on a cutting surface. Using a crafts knife, cut out a curved section from each head so the head curve smoothly fits the curve of the body.
Insert a toothpick halfway into each head, centered on the inside of the neck curve. Push the rest of the toothpick into a body, aligning the flat sides of the plastic-foam balls. Hot-glue each heads to the bodies; let dry.
Paint the spiders with black acrylic paint; let dry.
Push the ball-head pins into the front of the heads for the eyes.
Cut the chenille stems in half. Push one end into the underside edge of each spider body and bend outward for a leg. 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, as shown in the photo.
Cut two 9-inch lengths from the black feather boa for each spider. Glue one length in a spiral on the back and the other length on the tummy of each spider.