Lizard heads called the parietal eye, a basal ("primitive") feature also present in the tuatara. This "eye" has only a rudimentary retina and lens and cannot form images, but is sensitive to changes in light and dark and can detect movement. This helps them detect predators stalking it from above. Until 2006 it was thought that among lizards, only the Gila monster and the Mexican beaded lizard were venomous. However, several species of monitor lizards, including the Komodo dragon, produce powerful venom in their oral glands. Lace monitor venom, for instance, causes swift loss of consciousness and extensive bleeding through its
Mexican beaded lizard is listed on Appendix II of CITES. Fewer than 200 lizards remain in the dry forest habitat of the Motagua Valley, and this species of beaded lizard ("H. charlesbogerti") was facing extinction due to local extermination and loss of habitat for agricultural purposes. A conservation effort has been launched known as Project Heloderma to preserve the semiarid habitat of the Motagua Valley by the Nature Conservancy and partners such as ZOOTROPIC, CONAP, the International Reptile Conservation Foundation, Lincoln Park Zoo, Zoo Atlanta, and the San Diego Zoo. This effort has been successful in getting the Guatemalan government to list the