gecko gĕkˈō [key], small or medium-sized lizard of the family Gekkonidae. Some 1,500 species are distributed throughout the warm regions of the world, mostly in the Old World. Despite folklore to the contrary, their bite is not poisonous.

Many gecko species are arboreal, while others inhabit human dwellings. Most lack movable eyelids and have characteristic pads on the undersides of their feet that enable them to cling to smooth surfaces and to run upside down on ceilings. The pads contain microscopic backward-projecting hairs covered by tiny pads that may generate an adhesive force through van der Waals attractions (see intermolecular forces). Geckos are unique among lizards in that they possess voices, and different species make characteristic sounds. They feed on small animals, chiefly insects. Nearly all lay eggs.

The largest species is the 14-in. (35.5-cm) tokay, Gekko gecko, of SE Asia. The wall gecko, Tarentola mauritanica, of the Mediterranean region is commonly seen basking by day on walls and rocks; it hunts by night. The fish-scale gecko, Geckolepis megalepis, of N Madagascar has unusually large scales that readily tear away at the slightest touch and regrow in a few weeks. There are two native species in the United States, the leaf-fingered gecko, Phyllodactylus tuberculatus, of extreme S California and Baja California, and the banded, or ground, gecko, Coleonyx variegatus, of the deserts of the SW United States and N Mexico. The latter is a ground-dwelling form and lacks foot pads. In Florida there are several introduced West Indian species as well as the widely distributed Turkish gecko, Hemidactylus turcicus, originally from Africa.

Geckos are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Gekkonidae.

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