The Insect expansion! this game adds not only insects, but also some arachnids!
this mod is mainly for creative.
The best thing about this mod is that the bull ants will DIG! so if you make a terrarium and put some ants inside they should dig galleries underground!
termites will dig through wood and drop bonemeal! (they don't spawn in survival yet, i removed the naturally generating structure that spawned them in my closed beta version)
Earwigs will tunnel through the ground and into bull ant galleries!
Trapdoor spiders place a piece of clay that breaks when you walk on it!