Ballooning behaviors on the artificial platform.

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

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

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

Sketches of ballooning structures (body + ballooning threads).

<p>These structures were observed above the water surface, at heights of 1–8 m. Wind was blowing from left to right. Therefore, these structures were transported in the same direction as the wind. Black, thick points represent the spider’s body. Black lines represent ballooning threads.</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

Experimental material and place for 3-dimensional wind velocity measurement.

<p>(A) A 3-dimensional ultrasonic anemometer (Windmaster 1590-PK-020, Gill Instruments) is installed 0.95 m above the ground. (B) The simplest conditions (i.e., a flat surface) were selected. The flat place is covered with the 6 cm short cut grass. Within a radius of 300 m, there is no obstacle object.</p

Sequence of active sensing motion with front leg (leg I) (negative images).

<p>(A) The spider first senses the condition of the wind current only through sensory hairs on its legs. (B) Then, if the condition seemed appropriate, the spider sensed more actively by raising leg I and keeping this pose for 8 sec. (C) If the spider decided to balloon, it altered its posture. (D) The spider rotated its body in the direction of the wind and assumed tiptoe posture.</p

A crab spider’s ballooning process (images were converted to negative images to visualize ballooning lines).

<p>(A, B) Initial phase of spinning ballooning lines; (C, D, E, F) Fluttering of a bundle of ballooning lines. Because of turbulent flows in wind, the ballooning threads fluttered unsteadily. (G) Takeoff moment. (H) Airborne state of a ballooning spider. (Original video: see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004405#pbio.2004405.s011" target="_blank">S3 Video</a>).</p

Takeoff process for tiptoe ballooning.

<p>The probabilities are calculated based on the total number of behaviors at each stage (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004405#pbio.2004405.g005" target="_blank">Fig 5B</a>).</p