Most stingrays have one or more barbed stings on the tail, which is used "only" for self-defence. The sting may reach about 35 cm, and its underside has two grooves with venom glands. The sting is covered with a thin layer of skin, the sheath, in which the venom is held. A few members of the suborder, such as the manta rays and the porcupine ray, do not have stings.