Moorish Gecko

Not with suction or glue, but with a dry physical grip. The underside of each toe carries broad pads covered in millions of tiny hairs, and each hair branches at its tip into hundreds of even smaller spatula-shaped endings. When the gecko presses its foot down, these endings make contact with the surface at a molecular scale over an enormous total area, and the sum of the resulting weak attractions, known as van der Waals forces, is strong enough to hold the animal's weight many times over. The grip switches on and off as the gecko rolls its toes down and peels them up with each step, which is why it can run across glass or hang from a ceiling and still move quickly.

It is harmless and, for most people, worth keeping around. It has no venom, its bite is too weak to break skin in the rare event it bites at all, and it carries no significant disease risk in normal circumstances. What it does is eat mosquitoes, moths, flies, cockroaches, and other insects, particularly around outdoor and window lights at night, so a resident gecko is a free, quiet form of pest control. The only real downsides are the small dark droppings it leaves on walls and sills and its clicking call at night. If a gecko does need to be moved, it can simply be caught gently and released outside on a wall.

Most geckos avoid daylight to escape heat, water loss, and visually hunting predators such as birds, and they have vertical pupils and other features suited to night vision. The Moorish gecko has kept those night adaptations but has also become comfortable being out in the day, especially to bask and warm up on sunlit stone and to hunt in the shade of walls and eaves. Living on and around buildings, where there is always nearby cover in a crack or under a tile, probably makes daytime activity safer for it than it would be in the open, and being able to use both day and night lets it feed for more of each 24 hours.

It is native to the countries around the western and central Mediterranean and to the Atlantic coast of Portugal, Spain, and north-west Africa, where it has lived on rock and, for a very long time, on human structures. Over the last century or two it has also been moved by people, hidden in loads of plants, pots, roof tiles, and building stone, and it now has established introduced populations in parts of the United States, notably California and Florida, in Uruguay and Argentina, and on a number of Atlantic islands. In its native range it is one of the most familiar reptiles; where it is introduced it usually stays close to towns and ports.