Hi (Psi),
let's take a step back and look at the real "bugs":
Thinking about bugs, what makes them frightening? Do you fear beetles? Or flies? No. Bees and wasps? Probably not. How about spiders? Only if you have problems with your mother or they are really huge.
Now consider this:
You are a poor farmer, working hard every day on the either dry or flooded ground to plant grain. You hardly get sufficient to feed your starving family and save enough for the next years seed. And then, some bad day you see the sky darken. Legions of small, ugly things with brown-black hulls pitch into your meagre field - and you simply cannot do anything about it. Sure, you can squash one of them easily, although even a single one can take some damage and carry on feeding, but for each killed there are 100 waiting to take its place. You and your family will starve to death. Why do they come? Your priest told you something about an angry god, nothing you could do about anyway.
Can you imagine the horror?
Now consider this:
You are a poor colonist, working hard every day, but things aren't running good, the energy to run the machines might no be sufficient. And then, some bad day you see the sky darken. Legions of ugly things with brown-black hulls attack your town. These things are nearly as big as an orang-utan - and maybe as intelligent. First, they run in every trap the militia put up, but they quickly adapted. But while they got more cautious, they still didn't hesitate, when sacrificing their own would bring them an advantage. At height of the assault, the militia easily killed hundreds of them, although even a single beast could take quite a beating. But they keep coming. Finally, the overrun the militia, and the following slaughter is the most horrible you have ever seen - and will ever see, as they take no prisoners. You cannot communicate with them, no chance for peace negotiations. They eat the dead and put their hungry little larvae into the dying. Who sent those monsters? You don't know, maybe an angry god or some other power you don't know about or can do anything about. They aren't some kind of test or challenge - they are a punishment - your nemesis.
So, I guess:
- they are strange and ugly: Their body is distinctly different from ours, you can't talk to them, you cannot easily comprehend their actions.
- they are stupid but follow a clever plan: By human conventions, they are nothing more but animals - clever animals maybe, but still animals. So by all means, you should be able to outsmart them. But you aren't.
- they are Legion: No matter how many you kill of them, there are much more.
- they are tough: Even if you put enough metal into them to kill a whale, they eagerly keep crawling toward you, trying to kill you while dying.
- they are inhuman cruel: They will neither spare their Artgenossen (member of the same species) nor themselves - and certainly not you.
I wouldn't add too many castes with too specialised powers - this makes them colourful. Maybe they have even only one sex, fertilizing themselves. Only one to start a new population.
Maybe they are the dominating species on a single planet (a la "Pitch Black") or some kind of bio-weapon (as hinted in the "Aliens"-movies). Maybe the collectivity of them make up a hive-mind (as the "Tyranids" in GW WH 40K).
I like them.
Theophagos
Scary stuff.
