origin of the creatures

I hope these movies don't go for so long that they feel everything needs to be resolved and explained. I like the creatures being a mystery.

But what I think is scary about them that would need explained if they delved too much further. These creatures aren't the big bad. They're a tool of the big bad. They seem rather single-minded in their need to hunt and kill. While they don't seem to attack each other on sight, they don't really seem to hunt in packs either, they seem more solitary. I'm sure they can in their own way, but they don't seem to be overly communicative with each other either. And that they rely on sound and have no sight gives me doubts about their own capabilities of space travel.

What I'm getting at is that they're just sort of a first wave. They're pawns. The higher-up aliens, who are in charge of it all and flew them there, they're just using these creatures to clear the human population before they come down to claim Earth.