Re: Why do spiders have varied leg lengths

This is a very good question!  Generally the front legs of spiders are 
longer than the rest because of sensory structures that allow most spiders 
(those lacking good vision) to sense their environment.   There are 
exceptions.  Philodromid crab spiders have longer second legs.   This 
reached an extreme in the genus Ebo, species of which have second legs 
twice as long as the front legs.  These may be also because of sensory 
concerns, the spiders often sidling sideways.  Some jumping spiders (which 
have excellent vision for a spider) may have the third pair legs longer 
than the rest.   These are used (in males) for courtship display purposes.
The third pair in most spiders is the shortest (see black widows for an 
example).   I suspect that the other reasons for leg length differences in 
web-building spiders may be mechanical and related to actual web building 
and walking on silk.


Reference:

Foelix, R. F. 1996. Biology of Spiders. Oxford University Press, New York.