Animal locomotion on the surface layer of water

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 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 extert. Creatures such as the basilisk lizard 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. Water striders in particular are known to have feet that have minute hairs on them.

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 and a weight of about 40 millinewtons.

Water striders generate thrust by shedding vortices in the water: a series of "U"-shaped vortex 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 (note that some momentum is transferred by capillary waves; see Denny's paradox for a more detailed discussion)

Meniscus climbing
To pass from the water surface to land, a water-walking insect must contend with the slope of the meniscus at the water's edge. Many such insects are unable to climb this meniscus using their usual propulsion mechanism.

David Hu and coworker John W. M. Bush have shown that such insects climb meniscuses by assuming a fixed body posture. This deforms the water surface and generates capillary forces that propels the insect up the slope without moving its appendages.

Hu and Bush conclude that meniscus climbing is an unusual means of propulsion in that the insect propels itself in a quasi-static configuration; without moving its appendages. Biolocomotion is generally characterized by the transfer of muscular strain energy to the kinetic and gravitational potential energy of the creature, and the kinetic energy of the suspending fluid. In contrast, meniscus climbing has a different energy pathway: by deforming the free surface, the insect converts muscular strain to the surface energy that powers its ascent.