Figure 1.
Digital particle image velocity (DPIV) measurements of a running spider.
In the horizontal position, the laser light sheet is focussed 3 mm above the floor, at mid-height of the spider, just below the bottom eye row level. The yellow portion represents the camera's field of view. Spiders were gently triggered to run using a stick inserted through a small hole at the entrance of the wind tunnel.
Figure 2.
Horizontal flow field and close-up view of the flow around a running spider.
The sequence in (A) highlights the pockets of high air-flow velocity created by leg strokes superimposed on the air movements created by the body trunk movement. Neither the tips of the spider's legs, nor their associated flow patterns, are visible as they are located below the light sheet. The time delay between two images is 500 µs; the spider was running at a speed of 5.7 cm/s. The cartoon, adapted from [9], highlights the relative position of legs to body trunk. An overlay of two images (first image in white, second image in grey) of the moving spider, separated by 500 µs, is shown in (B). The zone of flow velocities above the measurable range is in black. The running speed was 10.5 cm/s.
Figure 3.
Vertical flow field and close-up view of the flow around a running spider.
The sequence in (A) highlights the high air-flow velocity above the spider's body. The time delay between two images is 500 µs; the spider was running at a speed of 3.7 cm/s. An overlay of two images (first image in white, second image in grey) of a moving spider, separated by 500 µs, is shown in (B). The horizontal component of the air flow in the near vicinity of the legs is always directed forward, as front legs do not move back and forth (see cartoon in Figure 2). The running speed was 21 cm/s.
Figure 4.
Flow velocity upstream of running spiders.
The observed speeds (mean and standard deviation; dots and error bars, respectively), and the fit of the statistical function (Eq. 2) are represented.
Figure 5.
Spider's attack speed and cricket escape time.
The potential escape time for a cricket (red line) is expressed as a function of the spider's attack speed. At slow attack speeds, the distance at which crickets can perceive spiders is limiting (ambush strategy), whereas at high hunting speeds, the escape time becomes limiting (cruising strategy). The potential escape time is defined as the time interval between predator perception by a cricket and hit by a spider running at a given speed. It is based on the distance, for a given speed, at which the threshold of 30 µm/s for danger perception is attained [13]. The minimal recorded escape time for crickets is around 0.2 ms (horizontal bar, [26]). The distribution of observed attack speeds and the five successful attacks (stars) were obtained from observations of real attacks, at constant speeds, during cricket-spider interactions [10].