An observational study of ballooning in large spiders: Nanoscale multifibers enable large spiders' soaring flight.

The physical mechanism of aerial dispersal of spiders, "ballooning behavior," is still unclear because of the lack of serious scientific observations and experiments. Therefore, as a first step in clarifying the phenomenon, we studied the ballooning behavior of relatively large spiders (heavier than 5 mg) in nature. Additional wind tunnel tests to identify ballooning silks were implemented in the laboratory. From our observation, it seems obvious that spiders actively evaluate the condition of the wind with their front leg (leg I) and wait for the preferable wind condition for their ballooning takeoff. In the wind tunnel tests, as-yet-unknown physical properties of ballooning fibers (length, thickness, and number of fibers) were identified. Large spiders, 16-20 mg Xysticus spp., spun 50-60 nanoscale fibers, with a diameter of 121-323 nm. The length of these threads was 3.22 ± 1.31 m (N = 22). These physical properties of ballooning fibers can explain the ballooning of large spiders with relatively light updrafts, 0.1-0.5 m s-1, which exist in a light breeze of 1.5-3.3 m s-1. Additionally, in line with previous research on turbulence in atmospheric boundary layers and from our wind measurements, it is hypothesized that spiders use the ascending air current for their aerial dispersal, the "ejection" regime, which is induced by hairpin vortices in the atmospheric boundary layer turbulence. This regime is highly correlated with lower wind speeds. This coincides well with the fact that spiders usually balloon when the wind speed is lower than 3 m s-1

Sand transport and burrow construction in sparassid and lycosid spiders

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Suspension of a point-mass-loaded filament in non-uniform flows: Passive dynamics of a ballooning spider

Spiders utilize their fine silk fibers for their aerial dispersal, known as ballooning. With this method, spiders can disperse hundreds of kilometers, reaching as high as 4.5 km. However, the passive dynamics of a ballooning model (a highly flexible filament and a spider body at the end of it) are not well understood. Here, we introduce a bead–spring model that takes into account the anisotropic drag of a fiber to investigate the passive dynamics by the various non-uniform flows: (i) a shear flow, (ii) a periodic vortex flow field, and (iii) a homogeneous turbulent flow. For the analysis of the wide range of parameters, we defined a dimensionless parameter, which is called “a ballooning number.” The ballooning number is defined as the ratio of Stokes’ fluid-dynamic force on a fiber by the non-uniform flow field to the gravitational force of a body. Our simulations show that the present model in a homogeneous turbulent flow exhibits the biased characteristic of slow settling with increasing turbulence. Upon investigating this phenomenon for a shear flows, it was found that the drag anisotropy of the filament structure is the main cause of the slow settling. Particularly, the cause of slow settling speed lies not only in the deformed geometrical shape but also in its generation of fluid-dynamic force in a non-uniform flow. Additionally, we found that the ballooning structure could become trapped in a vortex flow. These results help deepen our understanding of the passive dynamics of spiders ballooning in the atmospheric boundary layer

Ballooning behaviors on the artificial platform.

<p>Ballooning behaviors on the artificial platform.</p

Sequential relations between behaviors for ballooning.

<p>(A) The percentage frequency of the behavior transition (the total number of transitions: <i>N</i> = 141). (B) The transition matrix between behaviors (the total numbers of categorized behaviors: <i>N</i>_<i>I</i> = 25, <i>N</i>_<i>S</i> = 65, <i>N</i>_<i>T</i> = 41, <i>N</i>_<i>D</i> = 8, <i>N</i>_<i>H</i> = 2). The corresponding underlying data can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004405#pbio.2004405.s012" target="_blank">S1 Data</a>. B, takeoff; D, dropping and hanging behavior; E, escape; H, hiding motion; I, initial state; N, not flown; S, sensing motion; T, tiptoe behavior.</p

Experimental materials and methods for identification of ballooning lines.

<p>(A) A schematic view of wind tunnel tests. (B) Sampling of ballooning fibers in front of an open jet wind tunnel. (C) Reel with a steel wire to measure the length of ballooning silks.</p

Identification of the number and thickness of ballooning fibers through FESEM.

<p>Identification of the number and thickness of ballooning fibers through FESEM.</p

Scanning electron microscopic images of ballooning lines and drag lines.

<p>(A) Ballooning fibers of <i>X</i>. <i>cristatus</i> (1,300×). (B) Ballooning fibers of <i>X</i>. <i>audax</i> (10,000×). (C) Middle part of ballooning fibers of <i>X</i>. <i>audax</i> (20,000×). (D) Ballooning fibers of <i>X</i>. <i>cristatus</i> (30,000×). (E) One pair of drag fibers of <i>X</i>. <i>cristatus</i> (a weight of 18 mg) (20,000×). (F) Two pairs of drag fibers of <i>Xysticus</i> spp. (a weight of 15.6 mg), which attached together (20,000×).</p