In advance: Make fondant nails. Tint portion violet; reserve some white. Roll out 1/8 in. thick as needed. Cut 2 in. long tapered nails (1/2 in. wide at rounded bottom). Set lengthwise across cornstarch-dusted dowel rods; let dry overnight. Use royal icing to pipe tip 2 dot spider body and tip 1 dot eyes and outline web and legs. Outline and fill-in tip 2 icing face. For sugared nails, work with one color at a time. Brush on stripes with piping gel and sprinkle on sugar; let dry five to 10 minutes before applying next color. Let dry.