QUOTE(hazzard @ Dec 7 2005, 11:07 AM) [snapback]965176[/snapback]
Yes they do.
Spiders, unlike insects, have only two body segments instead of three; a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (called the opisthosoma), supported by a hard exoskeleton composed mainly of chitin.
'Really big' is a relative term. Before the dinosaurs appeared, giant insects and arthropods roamed the earth. Dragnoflies the size of crows, and spiders as big as house cats were present. But chitn, though storng, is not strong enough to support man-sized bugs. And the oxygen-content in the modern Earth is insuffucinet to support the primitive breahting apparatus of a large insect/arachnid.