Q: I recently turned 40 and have noticed spider veins forming on the outside of both thighs. What causes this? A: Spider veins, medically termed telangiectasias, are blue or red capillary veins visible just under the skin surface. Most common in the legs or around the ankles they can appear isolated as solitary fine lines or as branches or webs. They do not turn into larger varicose veins but usually become more numerous or widespread over time. Spider veins are very common