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 with large specimens exceeding . Maximum weight known is Although males are slightly larger than females, the lizards are not sexually dimorphic. Both males and females are stocky with broad heads, although the males' tend to be broader. The beaded lizard's scales are small, beadlike, and not overlapping. Except for the lizard's underside, the majority of its scales are underlaid with bony osteoderms. Their base color is black and marked with varying amounts of yellow spots or bands, with the exception of "H. alvarezi", which tends to be all black in color. The beaded lizard has a short tail which