No one who has watched a waiting spider suddenly race to a snared insect can doubt that web vibrations have a meaning. Jiggle the web yourself, and you reinforce this impression. The spider often ignores such human teasing.
Yet scientists have known for a century that spiders attack a tuning fork when it touches the web. Apparently, certain kinds of vibrations are more significant than others.
With the help of laser technology, scientists now are beginning to tap in on the kinds of messages involved. Any probe mechanically coupled to the web would distort the system. A laser beam does not have this disadvantage. Like police radar, a laser system measures web strand vibrations from the slight shift in frequency of the light reflected by the moving strand.
At Konstanz University in West Germany, W. Mitchell Masters and Hubert Markl have used such a system to study web vibrations in three dimensions. In the experiments they report in Science, they have worked only with orb webs -- the circular structures with radial strands running outward from the center.
They note that the spider usually waits at the hub of this network where the radii converge or hides nearby with a signal strand running to the hub with which it can monitor web vibrations. Thus the radii seem the most important communication elements, especially since the spider orients along them in locating a vibration source.
Masters and Markl measured a strand's vibrations longitudinally along its length, laterally from side to side in the plane of the web, and transversely up and down at right angles to the web. Longitudinal vibrations suffered virtually no attenuation up to frequencies around 3,000 Hertz (Hz) and were even slightly amplified above that. The other two modes were severely damped.
This suggests that longitudinal vibrations are the most important for a spider. Indeed, spiders have been found to respond more readily to such vibrations in some experiments. As for helping a spider zero in on prey, the two scientists found longitudinal vibrations also give the clearest directional signals.
This raises the question, yet to be answered, as to whether or not an orb web spider can tune out certain vibrations and selectively "listen" to longitudinal vibration only. Also, can it distinguish between different frequencies?
The findings may also clear up an older puzzle. The spider's vibration sensing organ is most sensitive at frequencies above 1,000 Hz. Yet signals transmitted from trapped insects seem to have little energy above 500 Hz. Now that they know higher frequency vibrations are readily transmitted, the scientists say the spider may readily detect such vibrations, weak though they may be.
The orb web seems to be a more marvelous instrument than one might imagine.