Spiders take flight with the smallest breeze by first detecting the wind and then spinning dozens of nanometer fibers up to seven meters long, according to a study published June 14 in the open access journal PLOS Biology by Moonsung Cho, Ingo Rechenberg, Peter Neubauer and Christoph Fahrenson at the Technische Universität in Berlin. The study provides an unprecedented detailed view of the "swelling" behavior that allows certain spiders to travel in the wind for hundreds of kilometers.
Many types of spiders get involved in hot air balloons, either to disperse from their place of birth, to look for food or couples, or to find new sites for colonization. While most balloon spiders are juveniles or small adults, less than 3 millimeters in length, some larger adults also become inflamed. Although the behavior has been studied previously, these authors are the first to make detailed measurements of both the sensory behavior and the silk fibers used to trap the wind.
Using a combination of field observations and wind tunnel experiments, they found that large crabeater spiders (Xysticus species), approximately 5 mm long and weighing up to 25 milligrams, actively assessed wind conditions by repeatedly raising a or both front legs and orienting to the direction of the wind. At wind speeds below 3.0 m / sec (7 mph), with the relatively light updrafts, the spiders separated multiple inflatable silts 3 meters long, before freeing themselves of a separate silk line that anchored them to the blade of grass from which they were thrown. A single spider released up to 60 fibers, most of them as thin as 200 nanometers. These fibers differ from a drag line, which is known as a balloon line, and were produced by a separate silk gland.
The authors concluded that balloon spiders actively detect wind characteristics and launch only when the wind speed and updraft are within relatively narrow ranges, increasing the probabilities of a productive flight. According to the dynamic fluid calculations that the authors made using their wind tunnel data, the spider depends on the updrafts that form in the light winds in which they are launched, ensuring even more a successful flight.
"The pre-flight behaviors we observed suggest crabeater spiders are assessing weather conditions before they take off," said Cho. "Balloon flight is not just a random toss to the wind, but one that occurs when conditions favor a productive journey."
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Cho M, Neubauer P, Fahrenson C, Rechenberg I (2018) An observational study of the formation of balloons in large spiders: nanoscale multifibers allow a flight of large spiders. PLoS Biol 16 (6): e2004405. doi.org/10.1371/journal.pbio.2004405