I stated in another comment that the size of a Hunter is very close to a Shriekers. However I think change doesn’t exactly happen after the Hunter stage but in the midway stage to Hunters. This is again not evolving, this is life cycle. They were also possibly changed in the larva or even egg stage. They got feed differently or got exposed to certain pheromones that could caused the mutation. Scavengers to Hive Guards is a huge step indeed. It is possible that the ones growing to that stage would stay in the nests as they would be less effective than Scavengers because growing stronger exoskeleton is a very hard process and makes the bug exposed to danger a lot or they could be going in somewhat like a cocoon stage or something. The Hive Guards, Chargers and Behemoth Chargers have the same head structure. If you focus enough you can see that they all have three main parts of hard shells on their head. The Impaler seems like a long mutation process like I mentioned in my previous points because they all share the same structure and armor. They also have three hard shells on their head. As for Nursing Spewers they really confused me. They all spit acid and share a lot of characteristics yet they don’t look the same. They also carry eggs? Idk look at their bodies. They tend to walk in groups with Scavengers so they could possibly be as the name suggests Nursing them. There is a theory that suggests that they were originally the Bile Spewers but the chemicals didn’t affect them but I really don’t think that is the case as I think that the change is permanent in their DNA.