Field observations

Crab spiders (Xysticus cristatus, X. audax, etc.) were collected at Lilienthal Park and along the Teltow Canal in the Berlin area, Germany, and observed each autumn, especially during October, from 2014 to 2016. Lilienthal Park was selected for the observation of preballooning behavior because the ballooning phenomenon of adult and subadult crab spiders is frequently observed in this region.

On sunny and partly cloudy days in autumn, 14 crab spiders (8 females, 2 males, and 4 not identified; adult or subadults) were collected at Lilienthal Park. These spiders were released in the same place on a self-built artificial mushroom-like platform (a 5.5 cm diameter half sphere, 1.2 m above a ground surface, gypsum material). This platform was intended to stimulate tiptoe preballooning behavior. Because of its convex surface (half sphere), spiders can easily recognize that the top of the convex surface is the highest position that may promote tiptoe behavior. The white color of this platform allowed the visual clarity of the spider’s behavior. During the observation, the ballooning behaviors were recorded by digital camera. Additionally, titanium tetrachloride was used for the flow visualization of the wind. The local wind speed and temperature were not measured directly, but the values from the Dahlem weather station (4.5 km distance from the observation site, the anemometer is installed 36 m above the ground in the 20 m-high forest canopy) may provide approximate conditions.

The 14 crab spiders were set a total of 27 times onto the mushroom-shaped platform in natural weather. The detailed preballooning behaviors in “takeoff” observation were analyzed statistically: (i) active sensing motion, (ii) tiptoe motion, (iii) tiptoe takeoff, (iv) drop down, and (v) rafting takeoff (see “Takeoff” in the Results section). If they did not tend to balloon or hid for about 5 min, they were recollected. Once a spider raised 1 or both front legs (leg I) and then put them back again on the platform, this was considered to be an active sensing motion and was counted as 1 behavior. Tiptoe motion, raising the abdomen and putting it down again, was also counted as 1 motion. The transitions between behaviors were also counted. These numbers were first expressed as percentage frequency by normalizing with the total number of transitions (see Eq [1], f_i,j is a normalized frequency of the transition from i-th to j-th behavior. n_i,j is the number of the transition from i-th to j-th. m is the number of categorized behaviors). The probability from one behavior to next behavior is expressed by normalizing with the total number of previous behaviors (see Eq [2], p_i,j is the probability of the transition from i-th to j-th behavior).

Gliding of crab spiders was observed in the autumn along the Teltow Canal because of the ecological and topographical benefits for the observation of ballooners: (i) crab spiders frequently glide along the canal during the autumn; (ii) the angle of the sun’s rays in the morning was appropriate for the detection of the ballooning silks during their flight; (iii) the dense trees on the opposite side of the canal provide a dark background, which facilitates the observation of floating silks. The shape of the threads during flight could not be photographed nor recorded as a video because only limited parts of the ballooning threads were reflected by the sun’s rays. However, the movement and shape of the threads could be recognized with the naked eye, and these shapes were quickly sketched by hand.