Love the backstory. That's great stuff, and if you can spill that information out in little bits, you can slowly but surely terrify your players. Awesome.
No real comments on their main stats, beyond the "yeah, the bite should be less for the juvenile." My concern as a GM would be implementation. These seem like guys that are designed to show up in an enormous load of badness, a swarm of them attacking the players all at once, and you seem to have created enough special abilities that I'd hesitate to use monsters this complex in large numbers. Do you have a computer utility to quickly make these guys and keep track of their special stuff for you, or did you plan on making a few archetypes and using those for most of the groups?
Otherwise, good stuff. In an alien-esque campaign, that'd be a good ugly bad guy to run into.