[Lord Aulianas Darakathos]

Alright, let's see what my sparse fluff skills can add to this argument.

I've always seen the Hive Mind as an ant colony, but with limitations. It is possible that they were antlike, but the queens gained the form of semi-sentient instinct that they now have. As they eventually evolved and ate everything on their planet, it might have registered within their proto-minds that 'we need off this planet before we starve.'

The idea that the Hivemind consists of every Tyranid is... implausible, as otherwise the 'node' effect would be unneeded. On top of this, that would mean that, due to territorial and self-preservation instinct, they could very well start attacking each other.

This, along with the 'Belly of the Beast' short story, in which the Hive Mind directly contacts a SM librarian scout, implies that there is some form of damn near omniscient mind in the background, such as the Starcraft Overmind.

As for the Shadow of the Warp thing, it might just be that the Hive Mind makes every Tyranid, even gaunts, low-level psykers, too low level to do anything more than communicate with other Tyranids. This, in turn, is also supported by the Ciaphas Cain books, in which Jurgen disorients any nearby Tyranids, on account of his Blank-hood. This massive concentration of psykers, even as weak as they are, could very well cause the 'ripple' effect, as if one had tossed a stone into a pond. This could disrupt any and all astropathic communication, as the sheer amount of stones being thrown would cause too much interference.

This also might explain why the Warp is so Chaotic... after all, every Eldar is a psyker, the Tyranids are stones, and human psykers are more powerful than either the Eldar or the Tyranids, but have a LOT less psykic discipline than the Eldar.

Oh, and the all-psyker theory is boosted by one word: Zoanthrope.


EDIT: Im a Necron player. Why do I know so much about Tyranids?