Page 108 - The Miracle in the Spider

108 THE MIRACLE IN THE SPIDER

can then locate its prey.
The spider's legs are the organs best endowed with these sensory
hairs. The hairs are hollow, and of rigid construction. The animal can
sense the origin of the vibrations emanating from a source of noise up to
a metre away. Furthermore, there is another sensory system sensitive to
temperature in the hairs on its legs. Then there are bald spots on the
surface of its body with enormously sensitive nerve endings inside. On
account of all these properties, spiders can sense any movement going on
around them or the approach of any body, even on their own skin.
If a spider loses a leg, it grows a replacement a while later. The new
leg will be shorter than the original one. The spider will not use this leg,
which does not even touch the ground, for walking. In fact, the spider can
walk quite comfortably with only half its original complement of legs,
namely four. The only reason for another leg to grow, albeit a short one,
is that the spider has need of the sensory hairs on it.
Spiders' sensitivity to vibrations on their webs is so well developed
that they can tell whether the source is prey caught on the web or a male
spider coming to mate.
Until a few years ago, it was thought that webs, because of their
elastic construction, could not transmit vibrations. But research, using the
newly developed machines called the "Doppler Laser Vibrometry," shows
that the situation is quite the opposite. It is now known that webs conduct
vibrations, despite their elastic construction, and that they increase the
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level of the vibration. However, no scientific reason for this has yet been
discovered.
The spider can very clearly perceive any kind of warning, from a tiny
sound wave to vibrations on its web. This extremely useful early warning
system which passes over the web, is a mechanism having the most useful
characteristics from the point of view of the spider. If we consider the fact
that each one of the thousands of hairs on the spider's body is attached to
a nerve ending and thence to the brain and that the spider can rapidly
evaluate the warning signals it receives, the complexity of the system will
become more apparent.