Supdon, Which aliens, the Nephilim? In which case, no. We don't know enough about the Nephilim to assume they DON'T have any variations of their race that exist like this, in a paracitic form, and given what they are, I would strongly suggest that more than any other race it is possible - we've seen different looking bugs throughout the game, just as we saw all kinds of arachnids in Starship Troopers.
Just a thought.
Just a thought.