"Bugs Mister Rico! Millions of 'em!"
I don't know if Robert Heinlein's Starship Troopers was the first novel to include semi-mindless spacefaring insects as a threat to humanity, but they are the earliest of which I am aware. Since then, of course, bugs have been a fan favorite. They're icky, and dangerous, and scary. I love 'em. I am NOT including intelligent insectoid species in this grouping - just the really buggy bugs.
Rebel Miniatures Terror Mites
Khurasan Miniatures Parasachnid Bugs
Please note that if I were not trying to limit myself to just 15mm scale miniatures, I would be including the really incredible Starship Troopers Arachnids produced by Mongoose Publishing. Very scary buggies, but way too big for 15mm gaming.