Spider veins tend to develop on the lower extremities, including the ankles and anywhere on the legs. They may congregate around the knees. Some say that this happens when a person has a habit of sitting with their legs crossed. Regardless of where they occur, spider veins typically accumulate in small groups of red, blue, and purple squiggly lines. They may be very short in length or may spread out several inches from a center point. What gives them their name is the way in which spider veins occur in a web-like pattern.