Adhesive pads on the legs of several vertebrates and invertebrates are structures that allow them to walk on vertical substrates and even "upside down", sometimes on surfaces as smooth as transparent glass in a window.
Animals that have adhesive pads can not only hold with great force, holding up to 100 times their own weight, but can also run without difficulty on a wall. It is evident that this adaptation so useful for small animals that need to climb trees, hide under the leaves or seek refuge or food through the rocks, evolved independently more than once. Beginning
Generally the adhesion capacity of this type of structure is based on a hybrid system with a liquid and a mechanical component, although in some cases it is exclusively mechanical, for example in the case of lizards, the liquid component is absent. < / p>
There are two types of physiological adaptations that allow adhesion, there are the wet-type pads (where the main strength of adhesion is capillarity) and the dry type (where the main strength of adhesion seems to be intermolecular or van forces der Waals). In any case it is necessary to achieve the greatest contact surface. For this the animals that have this adaptation have developed pads with a high density of microvilli (geckos, ants, bees, cockroaches and grasshoppers) or covered with a very soft and flexible epithelium (flies, beetles and some reduvideos) that penetrates the microscopic slots of the surface where they walk. This last type of pads generally has accessory glands that secrete substances to further increase the contact surface and therefore the adhesion strength. Adhesive pads.
To avoid that the high adhesive capacity of these pads prevent the animal from walking with dexterity there are very efficient mechanisms that dynamically control the degree of adhesion.
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