Re: why the spider web is so durable?

spider webs are made from silk which the spider pulls from its spinnerets. 
There are several types of spinneret which produce different types of 
silk. A combination of silks are used in the construction of an orb web. 
the main frame is made with very elastic, non sticky threads that are very 
strong. Their strength and elasticity is due to the arrangement of 
proteins, pleated like an accordian. The sticky part of the web is made by 
a different type of silk which is covered in glue droplets. This silk is 
also highly elastic due to the glue, the elasticity is needed to prevent 
the web from breaking when prey runs into it.
A very useful textbook about spiders is "Biology of spiders" by Rainer F. 
Foelix (2nd edition). This tells you everything there is to know about 
spiders and their webs.  

some useful sites- http://home.istar.ca/~hewak/spider.htm http://www.ufsia.ac.be/Arachnology/Arachnology.html