Do-It now offers a variety of spline options for customizing soft plastic tubes. The new Spike 15 & Squiggle 6 Strand Splines work great with Do-It’s existing line of tube molds and inserts enabling you to give your favorite tubes new life. Whether it’s the Standard 18 Strand, the Spike 15 Straight or the Squiggle 6 Strand Spline you’ll have the tools needed to give the fish a new look they won’t be able to resist. The spline is what makes the tail. The insert screws into the spline and determines how hollow the bait is. This is a spline (tail insert) for your mold.