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Cheap Transport for Fishing Spiders (Araneae, Pisauridae): The Physics of Sailing on the Water Surface

Robert B. Suter
The Journal of Arachnology
Vol. 27, No. 2 (1999), pp. 489-496
Stable URL: http://www.jstor.org/stable/3706047
Page Count: 8
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Cheap Transport for Fishing Spiders (Araneae, Pisauridae): The Physics of Sailing on the Water Surface
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Abstract

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. Taken as a whole, the data indicate that sailing is a remarkably cheap form of transportation for Dolomedes, but that, at least at the edges of large bodies of water, it involves risks because it is directionally uncontrolled.

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