Ever resource conscious, the bacterium always first creates Skulks – they seem to provide the best results for the least investment. After aliens spawn as Skulks, they can change their form into the other four species described here. A gestation sac forms around them, and resources and information are drawn from the bacterium. The alien that emerges appears to have the same knowledge and evolutions as it did in its previous form. An alien must have access to enough resources to gestate into each species. Some complex forms also require two or more hives to be active.

The higher cost of more complex species leads to fascinating variation in tactics. Losing one Onos costs the aliens as much as a dozen (evolved) Skulks. Because they basically "cost" nothing, Skulks seem to be more reckless and willing to die to destroy their target than their more powerful, and more expensive, brothers. On the other hand, a single Onos can destroy an entire squad and base.