Abstract
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.
Highlights
Some spiders from different families, such as Linyphiidae, Araneidae, Lycosidae, and Thomisidae, can disperse aerially with the help of their silks, which is usually called ballooning behavior [1,2,3,4,5,6]
The local wind speed at 1.2 m above the ground must be much lower than these values
From our wind tunnel test, we found that the Xysticus genus uses tens of nanofibers for their aerial dispersal
Summary
Some spiders from different families, such as Linyphiidae (sheet-weaver spiders), Araneidae (orb-weaving spiders), Lycosidae (wolf spiders), and Thomisidae (crab spiders), can disperse aerially with the help of their silks, which is usually called ballooning behavior [1,2,3,4,5,6]. If spiders perceive appropriate weather conditions for ballooning, they climb up to the highest position of a blade of grass or a branch of a tree and raise their abdomen as if standing on their tiptoes, in order to position the abdomen at the highest level, before spinning the ballooning lines They release a single or a number of silks in the wind current and wait until a sufficient updraft draws their body up in the air. Another takeoff method is called “rafting,” in which spiders release the ballooning lines from a hanging position, relying on their drag line [7,8,10] (see S3 Fig) In these ways, some spiders can travel passively hundreds of kilometers and can reach as high as 4.5 km above sea level [11,12]. Due to the spider’s incredible aerial dispersal ability, the physical mechanism of a spider’s flight has been questioned for a long time, in public media and in scientific research [16,17,18,19,20,21,22,23]
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