Snakes that come out at night are usually poisonous. They have large belly scales. The scales on the head, however, are usually small (except cobra and krait). Most poisonous snakes possess only right lung. The poisonous snakes have compressed tails, long, grooved and canulised fangs and their bites leave an impression of two fangs. The venom consists of two types of toxins: neurotoxin and haemotoxin. Nevertheless, there may be some toxic bites from the harmless snakes. The hearing or sight is poorly developed. The chemical sense in most snakes is mediated by a specialized organ called vomeronasal organ or Jacobson's organ. Although snakes have no external ears, they are sensitive to vibrations of the ground. Their locomotion involving lateral undulations of the body is commonly called" serpentine locomotion". A healthy snake may shed its skin at intervals of two months or less. The copulatory organ in males is paired (hemipenes) and it lies at the base of the tail. The females in many species do not have the left oviduct. Courtship in snakes is initiated by the male and copulation may last from two to twenty hours. Sperm may lie dormant in the oviduct. Most snakes lay eggs. Eggs are white or yellow in colour and are usually laid under natural vegetation cover. Many snakes lay eggs in holes for which they scoop out sand or soft earth with their snouts. The incubation period usually varies from 30 to 70 days. Some species are viviparous and give birth to live young. Many snakes reach sexual maturity at two years of age. In larger species it may take four or five years.