Why Spiders Will Always Find You

Spiders are among the most vibration-sensitive organisms in the world, second only to cockroaches.

Spiders abound this Halloween season, but for those who wish to slip past unnoticed by a real spider -- good luck. New research has found that spiders are second only to cockroaches when it comes to detecting vibrations.
Hungry spiders can detect the quietest movements and air flow shifts. Stimulus forces in the .01 near-undetectable range are enough for spider stimulation, according to a new published in the Journal of the Royal Society Interface.
In fact, a spider's entire body is built to detect almost anything and anyone that might cross its path.
"The spider has more than 3000 strain sensors embedded in its exoskeleton at many different locations, but most of them are on the legs and the compound organs, like the vibration receptors, are near leg joints," co-author Friedrich Barth, one of the world's leading experts on spiders, told Discovery News.
Both he and lead author Clemens Schaber are neurobiologists at the University of Vienna. Along with colleague Stanislav Gorb of the University of Kiel, they used a process called white light interferometry to perform the first ever quantitative examination of the sophisticated micromechanics of spiders. This process combines light waves in an optical instrument, allowing for very precise measurements of the tiniest things, such as force on a spider strain sensor.
The spider's sensors consist of minute slits of the lyriform organs that receive information on local movements. The scientists determined that each slit's sensitivity was at the nanoscale level, gradually decreasing with decreasing slit length.
Schaber and his team focused their investigations on adult females of the large Central American wandering spider, Cupiennius salei, taken from their Vienna breeding stock. Given its size and impressive hunting talents, it's a favorite species for spider studies, and has been analyzed before.
This particular spider "does not build webs to catch prey, but is a nocturnal sit-and-wait predator," Schaber told Discovery News. "Our spider receives vibrations through the leaves of plants. Both on the plant and in the web, spiders (in general) will attack the stimulus source if the vibration amplitude induced is within a certain range and if it contains a biologically meaningful range of frequencies."
"If both parameters are far from being prey-like, a spider will not respond or escape," he continued.
Spiders may therefore detect the presence of a human or other animal, but unless the invader's movements mimic those of typical prey, the spider will probably not attack. With such a sensitive ability to detect vibrations, spiders would forever be wasting their time on useless hunts, were it not for their ability to fine-tune the incoming sensory information.
Biologist George Uetz of the University of Cincinnati and colleague Shira Gordon also recently studied spiders and found that when certain spiders are in the mood to mate, they drum unique sexy vibrations, preferably on leaf litter, to attract partners. Wolf spiders have a particularly showy display involving leg taps and body bounces.
It takes keen sensory perception for a spider to detect such movements out of the surrounding environmental din.
If a spider doesn't "feel" you, it can also see, smell and taste you. Schaber explained that spiders "have vision, sensitive for low light levels, but at low temporal resolution." Minute chemical-sensitive hair sensors on spider feelers, called pedipalps, can also receive odors. Female spiders release a sort of pheromone perfume that can attract males.
Aside from telling us more about spiders, the research could lead to improved bio-inspired sensors for use in medical, military, business and other possible industry applications.