Spider locomotion on the water surface: biomechanics and diversity

Abstract

Spiders in many families are capable of locomotion on the surface of water, a capability that, at its simplest, requires only a strongly hydrophobic integument and the same postures and motions that are used on land. Specialized aquatic gaits, in contrast, are only characteristic in the Pisauridae, Trechaleidae, Ctenidae, and Tetragnathidae. They are less common features of aquatic locomotion in Lycosidae, are only occasionally encountered in Salticidae, and are rare in Araneidae. Most of what is known about the biomechanics of these specialized gaits comes from research on fishing spiders (Pisauridae) and, because the physics and hydrodynamics are similar in many respects, on water striders (Insecta: Hemiptera: Gerridae). In what follows, I have concentrated on the biomechanics of propulsion in water-walking spiders and water striders because propulsion on the air-water interface was mysterious until the 1990s when researchers began seeking answers to the central question: What provides the resistance against which a spider or water strider pushes when it sweeps its legs backward? The answers, now nearly complete, include a) dimple distortion, b) drag, c) generation of vortices, and d) nanoscale brushing of the water surface by hydrophilic hairs.

Water surface locomotion in tropical canopy ants

“…For example, the hydrophobic tarsi of riparian Hydrophorus flies allow them to alight on water or vegetation (Burrows, 2013), and the modified hind legs of tridactylid grasshoppers facilitate large jumps from both land and water (Burrows and Picker, 2010;Burrows and Sutton, 2012). Many spiders also locomote equally well on water and land (Stratton et al, 2004); some use a modified gait when on water while others do not (Suter and Wildman, 1999;Suter et al, 2003;Suter, 2013). Although less common, a few entirely terrestrial insect taxa (e.g.…”

“…However, this assessment is preliminary because quantification of the relative contributions of drag, bow wave, surface tension and other forces was beyond the scope of this project. Likewise, we did not measure the differential wettability of ant body parts, which is another potentially important variable in their interaction with the water surface (Suter, 2013). Detailed force and hydrophobicity analyses have been conducted for other taxa (Hu and Bush, 2010;Suter, 2013) and provide good models for future studies of water surface locomotion in ants.…”

“…Likewise, we did not measure the differential wettability of ant body parts, which is another potentially important variable in their interaction with the water surface (Suter, 2013). Detailed force and hydrophobicity analyses have been conducted for other taxa (Hu and Bush, 2010;Suter, 2013) and provide good models for future studies of water surface locomotion in ants.The kinematic features of swimming in terrestrial arthropods are highly variable among taxa (Miller, 1972;Andersen, 1976;Franklin et al, 1977). Similar inconsistencies occur among ants.…”

Solving a novel confinement problem by spartaeine salticids that are predisposed to solve problems in the context of predation

“…When leaving the island by swimming, spiders slowly placed their forelegs on the water, pushed off from the island with their rear legs, moved completely out into the water in a spread-eagle posture and then propelled their bodies across the water surface by moving their legs in a stepwise fashion (see Suter 2013). When leaving the island by leaping, spiders landed on the water at a point about halfway across, and then swam the rest of the way.…”

“…For making these comparisons, it was essential to use species we knew were capable of readily moving across water surfaces (i.e. we had to avoid using species that would tend to sink in water or become stuck in the surface film; see Stratton et al 2004; Suter 2013). Salticidae is a large spider family (about 5,600 described species; Platnick 2014), but the nine species we chose for our experiments are representatives of some of the few salticid genera for which there is evidence of proficiency at crossing water surfaces.…”