Animal Locomotion On The Water Surface

Animal Locomotion On The Water Surface

Animal locomotion on the surface layer of water is the study of animal locomotion in the case of small animals that live on the surface layer of water, relying on surface tension to stay afloat.

There are two means of walking on water; the regime determined by the ratio of the animal's weight to the maximum vertical force that the surface layer can exert. Creatures such as the basilisk lizard (which can run on water) have a weight which is larger than the surface tension can support.

Surface living animals such as the water strider typically have hydrophobic feet covered in small hairs that prevent the feet from breaking the surface and becoming wet. Another insect known to walk on the water surface is the ant species Polyrhachis sokolova. The pygmy gecko (Coleodactylus amazonicus), due to its small size and hydrophobic skin is also able to walk on the water surface.

According to biophysicist David L. Hu, there are at least 342 species of water striders. As striders increase in size, their legs become proportionately longer, with Gigantometra gigas having a length of over 20 cm requiring a surface tension force of about 40 millinewtons.

Water striders generate thrust by shedding vortices in the water: a series of "U"-shaped vortex filaments is created during the power stroke. The two free ends of the "U" are attached to the water surface. These vortices transfer enough (backward) momentum to the water to propel the animal forwards (note that some momentum is transferred by capillary waves; see Denny's paradox for a more detailed discussion.)