The "superfacehugger" has always seemed quite an interesting element in the alien life cycle to me. It makes sense that to protect the future of the hive, a superfacehugger would carry not only a queen embryo for further procreation purposes, but an embryo for an adult to guard her until she had reached maturity & was capable of laying eggs. I personally don't consider such things as "praetorian facehuggers" as canon though, as there is quite obviously no readily apparent need for praetorians amongst the alien "society", nor is there any proof that they even exist within the context of the Alien film universe.