Locomotion on the Water Surface - Gaits & Modes

from the laboratory of

Robert B. Suter, PhD

Vassar College

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Rowing and Galloping

Fishing spiders, Dolomedes triton (Araneae, Pisauridae), propel themselves across the water surface using two gaits: they row with four legs at sustained velocities below 0.2 m/s and they gallop with six legs at sustained velocities above 0.30 m/s. Because, during rowing, most of the horizontal thrust is provided by the drag of the leg and its associated dimple as both move across the water surface, the integrity of the dimple is crucial. We used a balance, incorporating a biaxial clinometer as the transducer, to measure the horizontal thrust forces on a leg segment subjected to water moving past it in non-turbulent flow. Changes in the horizontal forces reflected changes in the status of the dimple and showed that a stable dimple could only exist under conditions that combined low flow velocity, shallow leg-segment depth, and a long perimeter of the interface between the leg segment and the water. Once the dimple disintegrated leaving the leg segment submerged, less drag was generated. Therefore the disintegration of the dimple imposes a limit on the efficacy of rowing with four legs. The limited degrees of freedom in the leg joints (the patellar joints move freely in the vertical plane but allow only limited flexion in other planes) impose a further constraint on rowing by restricting the maximum leg tip velocity (to about 33% of that attained by the same legs during galloping). This confines leg tip velocities to a range at which maintenance of the dimple is particularly important.

The weight of the spider also imposes constraints on the efficacy of rowing: because the drag encountered by the leg-cum-dimple is proportional to the depth of the dimple and because dimple depth is proportional to the supported weight, only spiders with mass > 0.48 g can have access to the full range of hydrodynamically possible dimple depths during rowing. Finally, the maximum velocity attainable during rowing is constrained by the substantial drag experienced by the spider during the glide interval between power strokes, drag that is negligible for a galloping spider because, for most of each inter-stroke interval, the spider is airborne.

We conclude that both hydrodynamic and anatomical constraints confine rowing spiders to sustained velocities lower than 0.3 m/s, and that galloping allows spiders to move considerably faster because galloping is free of these constraints.

For further information on gaits, consult	Suter, R. B., and Horatio Wildman (1999). Locomotion on the water surface: Hydrodynamic constraints on rowing velocity require a gait change. Journal of Experimental Biology 202:2771-2785.
To view a QuickTime movie shownig rowing, click on the button to the right ...	QT movie

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Jumping

Vertical jumps of fishing spiders (Dolomedes sp.) from the water surface have been presumed to be evasive behaviors directed against predatory fish. We used high-speed videography to analyze the jumps of fishing spiders and then constructed a numerical model to assess the effectiveness of these jumps in evading predatory strikes by trout. Jump height (mean = 3.7 cm) and duration (mean = 0.17 sec) were similar across spider masses (0.05 &endash; 0.66 g) but latency to jump increased significantly with mass. To accomplish jumps of similar height, more massive spiders had to generate more force during the propulsive phase of the jump than did smaller spiders, and the contribution of fluid drag to the total force used in jumping was substantially greater for large spiders than for smaller ones. Our model juxtaposing the jumps of spiders and the attacks of trout revealed that jump heights and durations were inadequate: only the most lethargic strikes by trout could be successfully evaded by jumping vertically from the water surface.

Sailing

Many pisaurid spiders inhabit the edges of bodies of fresh water and actively propel themselves across the water surface using both rowing and galloping gaits. They also sail across the water, taking advantage of the wind and their nearly frictionless interaction with the water surface. The physical interactions of Dolomedes triton (Walckenaer 1837) (Araneae, Pisauridae) with moving air, in a wind tunnel in which the floor was water, formed the core of the present investigation. Spiders in an elevated (sailing) posture were subjected to greater drag forces attributable to air motion than were spiders in a prone (non-sailing) posture and therefore were transported substantially faster than prone spiders. In the context of transport velocity, the benefit of adopting an elevated posture was substantially greater (relative to mass) for small spiders than for large ones, although even under the relatively steady flow conditions of the wind tunnel the velocities of the small spiders in the elevated posture were more variable than either small prone spiders or large spiders. The efficacy of adopting an elevated posture was a consequence of the steep air velocity gradient that existed above the surface of the water in the wind tunnel and that also exists above any pond over which the air is moving

For further information on sailing, consult

Suter, R. B. (1999). Cheap transport for fishing spiders: the physics of sailing on the water surface. Journal of Arachnology 27:489-496.

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Modified: February 2000

Comments: suter@vassar.edu