The right colour and pattern is essential to the survival of a spider, allowing it to both capture prey and avoid predators. Colour is produced by pigments underneath the spider's hard outer "cuticle" (skin) and their brightness is in part determined by the presence of a substance called "sclerotin", which is used to strengthen the cuticle. Areas where strength is required and sclerotin is abundant, such as the chelicerae, tend to be dark. Areas requiring more flexibility, such as the joints, tend to be paler. In addition to these pigments, many groups, such as jumping spiders, sport brilliantly iridescent colours
using microscopic structures on the body or hairs which refract light. In this way they can produce any colour imaginable, including metallic golds and silvers.The St. Andres Cross Spider can change the brightness of its yellow bands by expanding or contracting it abdomen. The changing shape alters the angle of light striking the covering of hairs, making the colour more vibrant when threatened.
The Two-spined Spider pumps special fluid through the outer layer of the cuticle when disturbed, producing rapid waves of subtle colour changes through the yellow and around the outer edges. Some species of crab spiders, as well as the Garden Orb-weaver, are able to change their overall colour slightly to blend in with their environment.