Animal locomotion on the surface layer

animal locomotion on the surface layer 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 keep afloat.

There are two means of walking on water: the regime is determined by the ratio of the animal's weight <math>mg<math> to the maximum vertical force that can be exterted by the surface layer (<math>\sigma P<math>, where <math>\sigma<math> is the surface tension and <math>P<math> is the contact perimeter of the animal's feet). Creatures such as the basilisk lizard have <math>mg/(\sigma P)>1<math> and are discussed on animal locomotion by water-walkers.

Surface living animals such as the water strider typically have hydrophobic feet covered in small hairs that prevent the feet from breaking the and becoming wet. Water striders in particular are known to have feet that have minute hairs on them.

There are at least 342 species of water striders, according to biophysicist David L. Hu (Nature, vol 424, p663, August 2003). As striders increase in size, their legs become proportionally longer, with Gigantometra gigas having a length of over 20 cm and a weight of about 40 millinewtons.

Water striders generate thrust by shedding vortices in the water: a series of "U"-shaped filiaments 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.