My cat eats flies moths spiders etc will this do her any harm?

as long as the bug isn't poisonous, it's fine. Even then, very few spiders are poisonous enough to cause serious harm to a cat. Stinging bugs may sting her, but she should be fine after the pain and swelling go down, and she'll learn not to mess with that type of bug anymore.

think of her as your own, personal, full-time, in-house exterminator ^_^

It depends what bugs she eats. Consult your vet for more information.