Respiration by the black widow spider occurs mainly in the abdomen. Their respiration is composed of books of lungs and their tracheae. Typically, the two respiratory organs at the front end of the spider are the book lungs, while the two in the back end are the tracheae. However, in some spiders this may be reversed and scientists find it impossible to determine whether spiders have tracheae, book lungs, or both due to the fact that they are covered by hard, protective platelets. Usually, the way the respiration process begins is with book lungs. "Book lungs are paired respiratory organs composed of 10 to 80 hollow leaves that extend into a blood sinus separated by small hardened columns." In order to breath, small slits open up in the abdomen which allows air to flow in. After having breathed in air, tubes deliver the oxygen directly to various tissues.