An Observational Study of Ballooning in Large Spiders: Nanoscale Multi-Fibers Enable Large Spiders’ Soaring Flight

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 species, spun 50 to 60 nanoscale fibers, with a diameter of 121 to 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.