Prepare and bake brownies according to package directions using an 8-in. square baking pan lined with parchment. Cool completely in pan on a wire rack.
In a microwave, melt chocolate chips; stir until smooth. Remove 1 tablespoon melted chocolate to a small bowl; reserve for attaching eyes. Add noodles to remaining chocolate; stir gently to coat. Spread onto a waxed paper-lined baking sheet, separating noodles slightly. Freeze until set.
Cut nine brownies with a 2-1/4-in. round cutter for spider bodies. Attach eyeballs using reserved melted chocolate. With a bamboo skewer or toothpick, poke eight holes in top of each spider for inserting legs. Insert a coated noodle into each hole. Store in an airtight container.