I wrote an adventure for a previous 3.5 campaign that had a network of interlaced carrion crawler tunnels - with 13 holes leading up to the surface - as the entrance to a vampire's underground lair. The PCs were hunting the vampire, so first they had to crawl into one of the holes, then wriggle around on elbows and knees (that's how cramped the tunnels were) throughout what was effectively a little maze, looking for the one passageway that led further down into the ceiling of the first chamber in the vampire's cavern network. And, of course, as the PCs are crawling around, the 3 or 4 carrion crawlers who live in those passageways (and who already know which tunnels lead where and which connect to others) are maneuvering around so they can attack them from behind, the direction where their weapons aren't pointing.... I made a few larger chambers in the crawler network where paralyzed victims would be dragged away to be eaten and have eggs laid upon their corpses, so the newly-hatched baby crawlers would have some already-decomposing meat to chew upon after breaking out of their rubbery eggs.
Johnathan
Johnathan