I have long thought about the many shapes and forms that the flood can take. It is amazing how many different creatures can be spawned from a single species. I have come up with the conclusion that the Flood are not restrained by only a few forms but can take any form that they desire, within their own limits of course. I back this up with the fact that the leader of the flood is the almost omniscient Gravemind who I believe has control over the morphological structure of his minions. For instance, the flood combat forms of Halo: Combat Evolved are hardier and more resiliant to enemy assault than those of the other games but they also take much longer to be formed. Gravemind, upon noticing this, could have slowly changed the morphology of the flood into a more combat ready form that, albeit weaker, is ready to transform hosts as soon as they are infected. This would also explain pure forms which are just mobile biomass that the gravemind has conjured up to back up his forces in the battlefield. I also believe that the large flood structure in the game Halo Wars was actually a gravemind. If the flood are as changing as I suspect than the gravemind himself could take any form he desires be it a large snake like creature for communication or a large armored dome-like structure to better defend himself. The flood forms that appear in Halo Wars such as swarmers and flood root could be pure forms that the Gravemind of this location conjured up to battle in a desolate environment. The Gravemind of the halo trilogy could have decided not to use these forms because they were unsuitable for the battle at hand and were therefore replaced by forms that would do better in this type of conflict. Who knows, maybe the gravemind of halo wars preferred his pure forms because they excelled at battling the forerunners but the gravemind of the halo trilogy changed them to better fight a new enemy.