As in the question.
It is possible that there may be two species which are so interdependent that one is necessary for the other to breed. (Zombie ants come to mind, but this would be on a different scale). I am interested in whether the relationship between facehuggers and the full xenomorph is a case of "facehugger is a part of the life-cycle of a xenomorph" or a case of "two inter-dependent species".
(By the nature of symbiosis, it is also possible that these two started as independent species and have combined into one super-organism.)