What I got what that these are just generic non-specific aliens. Still it is interesting that he chose Chitauri.Whedon set the record straight, since the film doesn't explicitly explain just who is helping Loki in his attempt at world domination. "The alien race are the Chitauri -- or a version of them -- because they are not one of the key races and they don’t have a storied history. That wasn’t the point. I know this debate will go on long after I’m dead. I would say it was the Kree-Skrull race."